Rear Clunk Around Corners - Recording Inside

[Carbonvert]

Hey Guys,

I am hoping you can help me ID a clunk from the rear end of my car.

During the off season some work was performed- relevant work at the rear end:

• Outer upper and lower ball joints
• Synco Design Works monoball RTAB
• Brakes
• The car was on jack stands for close to a month while work was being done

Rear End Work in the last 15k miles:

• Rear shocks
• RSM
• OEM Diff Cover for new bushings
• Sway bar End Links
• ECS Poly Front Diff Mount (I hate this thing but its still in good shape)
• Guibo

Car Background:

1. 2002 Convertible 6MT
2. 85k Miles
3. I'm the second owner for the last 11 years, the car has lived a pretty easy life
4. Rear floor/subframe has NOT been addressed yet

Issue:

• Since putting the car back on the road I have a knock that will occur in certain situations
  • The easiest way to reproduce it is during cornering at ~20MPH
  • You can reproduce the noise on command in a roundabout if you apply a bit of throttle
  • It is rare for this noise to occur during straight acceleration/deceleration
• The noise is a single knock/thud from the rear that can be felt under the car
• The noise CANNOT be reproduced when going over bumps

Investigation thus far:

• I have double checked all the torque values of all fasteners I removed during the most recent work
  • I made sure everything was torqued at ride height
• I checked the tightness of
  • All 3 diff bolts
  • Subframe bolts
  • Center Support Brace
• I looked closely for subframe cracks around the mounting points and from within the trunk
  • I realize I would only be able to see then here if the damage was extreme

Linked is a recording of the noise. In this recording I am going through a roundabout at ~20mph. The noise can be heard quietly at 11sec (10sec remaining in link) and very clearly at 16sec (5sec remaining in link)

http://sndup.net/29bkd

Any ideas what this could be? Obviously my mind immediately goes to the worst case - a really damaged subframe floor, but I don't have any creaking and this noise seems pretty extreme to come our of nowhere during the first drive of the season. Additionally this noise can only easily be reproduced when cornering. From what I've read a damaged subframe also makes noise in a straight line. Could sitting on jack stands for awhile be enough to take any minor cracks I had and make them extreme?

I planned on brining the car to an expert who can do a top and bottom subframe floor reinforcement next year

Thanks for any insight you can give

[ugaexploder]

I presume the noise wasn't there prior to all the work being done? Car sitting on jack stands shouldn't hurt the car. It's probably better for the car if anything.

I would say it's your diff...just an assumption but you tightening everything up by replacing some worn out bushings could have made this sound pronounced whereas before with the slop of old bushings and what not, it may have not been present.

Also, I've had this happen before on one of my older cars and it was when I was younger and didn't properly torque sway bushings with load on them. I tightened everything on the sway bar with no load so when I lowered it to the ground, it would make a clunky noise but only in certain situations.

[bmwfnatic]

I am going to say that this is the “M Clunk” produced by the GKN LSD core.

But it’s hard to diagnose things over the internet

[Slideways]

Did you make sure the springs are seated correctly? I have heard this type of clunk/binding noise from a rear spring that needed to be rotated in its seat to find an orientation where it would not create a clunking/binding noise.

[01SG]

Maybe that poly diff bushing has something to do with it.

[WestBankM4]

It's interesting you bring this post up because immediately after I took my car to have the full suspension refresh AND the Vincebar install by a well known shop in the Bay Area, I have the same knock. It's not the differential, it sounds to me like mine is body/suspension related but it makes a single thud when making a sweeping left or right at low speeds.

Slideways - Good point actually, I'm going to get under it today and see if the springs were not seated properly. I'm assuming the last coil should be rotated into the notch like every other car?

I hope you find the source of the issue OP because I have the same exact issue.

[heinzboehmer]

Grab a wrench and put it on every single suspension bolt you can get to. I had the lower control arm to subframe bolts back out slightly on my car and that caused a very similar noise.

[Slideways]

It's interesting you bring this post up because immediately after I took my car to have the full suspension refresh AND the Vincebar install by a well known shop in the Bay Area, I have the same knock. It's not the differential, it sounds to me like mine is body/suspension related but it makes a single thud when making a sweeping left or right at low speeds.

Slideways - Good point actually, I'm going to get under it today and see if the springs were not seated properly. I'm assuming the last coil should be rotated into the notch like every other car?

I hope you find the source of the issue OP because I have the same exact issue.

For example, the Ohlins rear springs need to have the lettering facing the outside of the wheel well; otherwise, there will be a clunking sound coming from the rear when loading that corner. KWs should have markings on the spring as well.

[WestBankM4]

For example, the Ohlins rear springs need to have the lettering facing the outside of the wheel well; otherwise, there will be a clunking sound coming from the rear when loading that corner. KWs should have markings on the spring as well.

I'm using ST XTA's, those are KW springs correct?

[Slideways]

I'm using ST XTA's, those are KW springs correct?

Yes, IIRC they all use the same rear springs.

[Notan_S54]

This is something I need to know now too! My car does this noise going over speed bumps and non flat (pothole like) areas. Driving me nuttssss.

[Carbonvert]

I presume the noise wasn't there prior to all the work being done? Car sitting on jack stands shouldn't hurt the car. It's probably better for the car if anything.
I would say it's your diff...just an assumption but you tightening everything up by replacing some worn out bushings could have made this sound pronounced whereas before with the slop of old bushings and what not, it may have not been present.

Also, I've had this happen before on one of my older cars and it was when I was younger and didn't properly torque sway bushings with load on them. I tightened everything on the sway bar with no load so when I lowered it to the ground, it would make a clunky noise but only in certain situations.

The noise didn't start until after the work - that's what gives me hope this isn't a catastrophic subframe failure

You think the diff would be influenced by lateral loads? In some cases I can reproduce this noise just by going around a bend, no throttle input at all

I tried disconnecting the end links just at the bolt that goes through the end-link bushing. It seemed to reduce the occurrence of the noise but that might just be in my head. Maybe I should remove the sway bar completely and take it for a ride

Did you make sure the springs are seated correctly? I have heard this type of clunk/binding noise from a rear spring that needed to be rotated in its seat to find an orientation where it would not create a clunking/binding noise.

I had the same thought but couldn't find any information. I am running OEM springs - is there a procedure for seating them? It seemed like they only go in one way

Maybe that poly diff bushing has something to do with it.

I've had that bushing for years - I hate the NVH, but its in good shape and the bolt is tight

It's interesting you bring this post up because immediately after I took my car to have the full suspension refresh AND the Vincebar install by a well known shop in the Bay Area, I have the same knock. It's not the differential, it sounds to me like mine is body/suspension related but it makes a single thud when making a sweeping left or right at low speeds.

Slideways - Good point actually, I'm going to get under it today and see if the springs were not seated properly. I'm assuming the last coil should be rotated into the notch like every other car?

I hope you find the source of the issue OP because I have the same exact issue.

Mine also doesn't sound like the diff. This gives me hope its not the subframe!

Grab a wrench and put it on every single suspension bolt you can get to. I had the lower control arm to subframe bolts back out slightly on my car and that caused a very similar noise.

I will check those- I have not checked any of the control arm - subframe bolts since those were not touched in the work I did

[Carbonvert]

Well the mystery continues.

I checked the tightness of everything. A local shop with a lot of e46 experience checked the tightness of everything and slightly dropped the subframe to inspect the subframe floor. No issues found.

I notice that if I have the brakes slightly applied it is much harder to reproduce this noise. Doing this would preload the axles.

I wonder if the axles could be going bad. They are not leaking but I’m sure the grease has been liquidifed at this age. It’s a very expensive part to just throw at the problem.

[sapote]

The noise didn't start until after the work - that's what gives me hope this isn't a catastrophic subframe failure

You think the diff would be influenced by lateral loads? In some cases I can reproduce this noise just by going around a bend, no throttle input at all.

I don't think it's the diff clunk in this case as it had no noise before you're working on the rear.

I would check the muffler mounts to make sure it doesn't shift L or R during a turn. I once had a friend drove my 1974 3.0CSi around the sharp corners with me laying in the trunk to pin point the noise, and it was the muffler curved pipe that hit the floor due to a weak rubber mount.

[Stilt]

Rtab bushings were installed properly and the housing was not crushed or deformed?