Diagnosing a new rear-end "popping" noise

[heinzboehmer]

I did, that's why I had it sorted. Had a reputable shop which does most of the E46 M3 RACPs in NZ fix mine too since I don't have much experience welding. I can't see any additional cracking but I also have yet to do a proper check. I'll check the axle bolts too.

Put a wrench on every suspension bolt while you're there. Recently had the lower wishbone to subframe bolts (on both sides) come loose on my car. Made a similar noise

[Thoglan]

I suppose its possible you had spot weld failures prior to reinforcement? What reinforcement kit did you go with?
I'm currently frustrated as I can't decide which direction to go with fixing this...a fully disassembled vehicle on the quick jack and it's difficult enough to find the time to do the simple repairs I was tackling an hour here and there. Guessing the Vincebar or CMP option is at least a full day or two's effort?

I did, that's why I had it sorted. Had a reputable shop which does most of the E46 M3 RACPs in NZ fix mine too since I don't have much experience welding. I can't see any additional cracking but I also have yet to do a proper check. I'll check the axle bolts too.

[fattycharged]

So the popping is caused by RACP separation? Or did you also have loose axle bolts?

My subframe is fully reinforced top and bottom and wheel arch seam welded and I'm sure I have heard this popping every so often.

I suppose its possible you had spot weld failures prior to reinforcement? What reinforcement kit did you go with?
I'm currently frustrated as I can't decide which direction to go with fixing this...a fully disassembled vehicle on the quick jack and it's difficult enough to find the time to do the simple repairs I was tackling an hour here and there. Guessing the Vincebar or CMP option is at least a full day or two's effort?

[eisen]

Great news for me, after this thread I thought I'd do the same inspection on mine after recalling that subtle "pop" sound over my left shoulder last time I drove it...I've got separation on driver side rear arch along the seam, though nothing visible inbound toward the actual rear mounting point with epoxied plates. I dont think I trust my welding skills on that thin edge, all I have is a TIG and it's been a while to jump back in upside down, so I'd really like to find out who you end up using for mobile as I'm in Houston as well.

Well, I guess I'm happy for this thread to serve as a PSA to other owners! And same for me, I have no idea how long mine has been like that, but hopefully both of ours result in that being the only damage. Unfortunately, I haven't had much of a chance to deal with anything on my car yet. This happened at a pretty inopportune time as we're moving to GA next month.

Side note, that front diff bolt drives me nuts. Mine seems to loosen after every track day and causes a lovely clunk. I finally ordered a Nordlock washer for it to see if that will keep it tight for longer than a couple months...

[fattycharged]

So the popping is caused by RACP separation? Or did you also have loose axle bolts?

My subframe is fully reinforced top and bottom and wheel arch seam welded and I'm sure I have heard this popping every so often.

Assumption now is the sound is RACP separation, though I have not driven it for quite some time. No axle bolts, or any other bolts, were loose. Prior to this thread, I assumed it was the RSM as I had fully blown out and worked V3's on still. The separation at the seam there was coated with road grime, there is obviously no corrosion present as it wasn't exposed, so I really have no idea how long it has been like that. Could have been a decade of no additional separation and I won't know how the interior looks till I open it up. I will say this, I am still in the preventative mindset with all sets of fingers crossed. Later this week I'll clean up the passenger side and look for separation. I really don't want to strip this and search for popped spot welds....

Plate install was done close to a decade ago, I don't have great documentation of the job unfortunately, though it wouldn't matter much as I only addressed directly at the mount points. There were not an abundance of cracks, just a few noticeable ones on that driver side IIRC. Now, today we realize I should have removed every ounce of coating to inspect, which I obviously did not.

E36 diff bushing that has been on the cover for at least the past 60k miles, still in good shape, not much more deflection than the new set in hand.

I'll accompany it with a photo of the noise that led me to get under there over a decade ago, so my fears were alleviated when I saw that it was just that 100lb diff slamming into the tunnel and not my RACP fully separated Front bolt issue that presented itself again a few years later...but its still in tact, so we'll give it another go.

[Thoglan]

Great news for me, after this thread I thought I'd do the same inspection on mine after recalling that subtle "pop" sound over my left shoulder last time I drove it...I've got separation on driver side rear arch along the seam

So the popping is caused by RACP separation? Or did you also have loose axle bolts?

My subframe is fully reinforced top and bottom and wheel arch seam welded and I'm sure I have heard this popping every so often.

[bigjae46]

Great news for me, after this thread I thought I'd do the same inspection on mine after recalling that subtle "pop" sound over my left shoulder last time I drove it...I've got separation on driver side rear arch along the seam, though nothing visible inbound toward the actual rear mounting point with epoxied plates. I dont think I trust my welding skills on that thin edge, all I have is a TIG and it's been a while to jump back in upside down, so I'd really like to find out who you end up using for mobile as I'm in Houston as well.

I do my own welding with a MIG welder. Not sure I'd trust myself either. I have a guy that can fix that at a reasonable price.

[fattycharged]

Great meeting you yesterday!

First step is to cut out and inspect the top of the front mounts so you can assess the damage. Then clean up the seam sealer around the cracks and a little beyond the cracks you can see.

I know we talked about welding - you will have to MIG weld any cracks. A lot of that welding is upside down which is far more challenging. I would find a mobile welder to do that. Most of the work and cost is the prep work to expose clean metal. You can do most of that as part of inspecting and assessing the damage.

Great news for me, after this thread I thought I'd do the same inspection on mine after recalling that subtle "pop" sound over my left shoulder last time I drove it...I've got separation on driver side rear arch along the seam, though nothing visible inbound toward the actual rear mounting point with epoxied plates. I dont think I trust my welding skills on that thin edge, all I have is a TIG and it's been a while to jump back in upside down, so I'd really like to find out who you end up using for mobile as I'm in Houston as well.

[bigjae46]

Urgent probably wasn't the right choice of words on my part, more like "catastrophic". I'm definitely glad I caught it relatively early and am weighing my options for topside reinforcement as we speak. Sorry to hear about your troubles, too, but glad it's all squared away now!

Man, I couldn't agree more. The shops I've talked to about my wheelwell have been shocked and said they've never seen anything like it. This car has had multiple track inspections, too, and I'd hope any M oriented shop would check for common/obvious things like that prior to signing off on it, but I digress. Really, this whole thread showcases that I should've just been more diligent myself 😬.

Great meeting you yesterday!

First step is to cut out and inspect the top of the front mounts so you can assess the damage. Then clean up the seam sealer around the cracks and a little beyond the cracks you can see.

I know we talked about welding - you will have to MIG weld any cracks. A lot of that welding is upside down which is far more challenging. I would find a mobile welder to do that. Most of the work and cost is the prep work to expose clean metal. You can do most of that as part of inspecting and assessing the damage.

[eisen]

Your entire rear left rear mount is pulling away from the car and not held in by much anymore, the next stage is for the panel to start cracking (as mine did), so to prevent further damage (which is much more difficult to repair compared to the stage it is in now, I would say it is somewhat urgent.

Urgent probably wasn't the right choice of words on my part, more like "catastrophic". I'm definitely glad I caught it relatively early and am weighing my options for topside reinforcement as we speak. Sorry to hear about your troubles, too, but glad it's all squared away now!

I find it amazing how many "BMW specialist shops" that supposedly know all about M cars here in Houston still neglect the top side. One in particular had no clue as to what I was talking about. I guess us forum people aren't so stupid after all.

Man, I couldn't agree more. The shops I've talked to about my wheelwell have been shocked and said they've never seen anything like it. This car has had multiple track inspections, too, and I'd hope any M oriented shop would check for common/obvious things like that prior to signing off on it, but I digress. Really, this whole thread showcases that I should've just been more diligent myself 😬.

[bigjae46]

From personal experience, I also have a previously plated car that was en-route to fail again.

You have basically popped all spot welds that kind of ‘let go’ peacefully, the 3 along the wheel arch and at least one visible near the left spring perch (usually at least one more but hidden by thick seam sealer).

Your entire rear left rear mount is pulling away from the car and not held in by much anymore, the next stage is for the panel to start cracking (as mine did), so to prevent further damage (which is much more difficult to repair compared to the stage it is in now, I would say it is somewhat urgent.

In addition to this damage at the rear, the front of the RACP was also pulling away on my car, with all 8 of the spot welds (4 per side) where the RACP attaches to the rear seat area having let go.

I am not trying to scare anyone but in my opinion based on my findings repairing my own M3, the topside reinforcement is very important.

I find it amazing how many "BMW specialist shops" that supposedly know all about M cars here in Houston still neglect the top side. One in particular had no clue as to what I was talking about. I guess us forum people aren't so stupid after all.

[bmwfnatic]

I'm still in need of topside reinforcement and to stitch weld that crack in the wheelwell, but at least it isn't urgent.

From personal experience, I also have a previously plated car that was en-route to fail again.

You have basically popped all spot welds that kind of ‘let go’ peacefully, the 3 along the wheel arch and at least one visible near the left spring perch (usually at least one more but hidden by thick seam sealer).

Your entire rear left rear mount is pulling away from the car and not held in by much anymore, the next stage is for the panel to start cracking (as mine did), so to prevent further damage (which is much more difficult to repair compared to the stage it is in now, I would say it is somewhat urgent.

In addition to this damage at the rear, the front of the RACP was also pulling away on my car, with all 8 of the spot welds (4 per side) where the RACP attaches to the rear seat area having let go.

I am not trying to scare anyone but in my opinion based on my findings repairing my own M3, the topside reinforcement is very important.

[sapote]

TLDR: A couple of the E torx bolts that secure the axle to the diff were loose.
.

Great. A completely loose half shaft at high speed will swing around, hit and damage things around. As the diff was removed before, so the bolts backing out were not torqued to the factory spec is my guess.

[bigjae46]

That’s good that you found the issue. I’m always paranoid that I missed a bolt so I usually double check them.

I have a new set of axle bolts if you need them. Send me a PM if you’re interested.

[eisen]

Well guys, I found the source of the noise, and it wasn't the RACP (hallelujah).

TLDR: A couple of the E torx bolts that secure the axle to the diff were loose.

I spent most of the day digging through the rear of the car looking for signs of any other cracks. Everything looked fine; I even scraped up some of the sound deadening in the trunk and couldn't find anything that could be causing a noise like that (thanks for making that point about it being in the early stages, sapote , it made me want to look for other evidence).

Finally decided to go back through and retrace my troubleshooting steps just in case. After messing around with the driver's side axle, I remembered reading online about those torx bolts from the axle to the diff coming loose on racecars. Decided to check mine just in case and, what do ya know, a couple were loose - one fingertight. Still wasn't convinced that could do it, but torqued them to spec, noticed that it got rid of a tiny bit of play that was there, lowered the car down, and voila - no more noise. What a rollercoaster. I'm planning to order some new ones, loctite them, and hopefully never deal with this again.

I'm still in need of topside reinforcement and to stitch weld that crack in the wheelwell, but at least it isn't urgent.