Clunking when on/off the gas quickly

[bimmer]

Any ideas on what this noise is? https://youtu.be/PHbrSnNtxQ4

Started last night after some spirited driving. It only happens when coming on and off the gas relatively quickly. It does not happen under braking, over bumps, or turning. My first thought was engine/trans mounts but just got under the car and they were still snug/couldn't find any play in them. They are solid condor mounts maybe a little over a year old. I just recently put in turner monoball FCABs and couldn't find any play in the control arms when down there either so I don't think they are the culprit and if they were would have felt it under braking/bumps. Any and all suggestions appreciated...driving me nuts.

[bimmer]

Ok I think I may have found it. Seems there is a small amount of play in the transmission. I assume in gear there shouldn't be any and that this isn't normal? See video here...the sound is very similar: https://youtu.be/9WhbQHotT-U

[bimmer]

Some additional info after a short test drive...when I clutch in while driving it makes the same noise as soon as the clutch is fully depressed

[duracellttu]

Typically this is due to either slop in the diff gears or loose diff mounting bolts. If those check out, then check the drive line: guibo and CSB. Lastly, it could be rear wheel bearings, but less likely.

[bimmer]

Actually just got back from the garage having had the same idea on the diff bolts. All three were still torqued to spec. I replaced the guibo and CSB within the last year/5000 miles. How would I go about checking for slop in the diff gears? Is there any way it could be the clutch? It is original as far as I know at 150K miles. It feels fine outside of the clunk when fully depressed thing.

[Epsilon]

If you roll your car back slowly while in neutral and you pull the ebrake to stop, does the rear end lift? I encountered what you felt and swapped a lower mile diff and problem gone. Definitely too much slop in the diff. Otherwise, it can also be the driveshaft or CV.

[Steve]

Have you ever replaced your front differential mount? I had some clunking coming from my touring recently and a front differential mount and new rear cover fixed it right up.

Spoiler alert, the job is awful but if you haven't done it at 150k miles, it is time.

[Phoenix///M3]

Fixed my clunk by replacing the two rear diff bolts with the upgraded (not full threaded) ones. It was an immediate fix. I loosened and re-torqued the front bolt at the same time.

https://www.turnermotorsport.com/p-7...t-23001222891/

[bimmer]

I've got the condor solid differential mounts. They probably have around 5k miles on them. I just had the rear subframe out 5-6 weeks ago and mounts looked fine at that time. I'm not sure if the rear bolts were upgraded or not...I'm going to hoist it back up hopefully this afternoon. I'll pull one out and check. If those are good, as much as I don't want to do it yet again, I'll drop the exhaust and do a thorough inspection of the driveshaft to see if maybe the CSB somehow came loose or if the UJ or CV joint is shot.

[bimmer]

So I just checked and have the old rear diff bolts. I have the new ones on order, although the current seem to be in perfectly fine condition...

[bimmer]

Indy confirmed today that the differential is very much shot and driveshaft should probably be replaced. Not surprising at 150K. The question I am struggling with now is to do a stock rebuild of the diff or upgrade to 4.1 gearing. I'm finding an additional $1100 on top of the $1400 rebuild hard to justify, but seems folks are very happy with the 4.1 for split track/weekend duty which is my situation.

[eacmen]

Indy confirmed today that the differential is very much shot and driveshaft should probably be replaced. Not surprising at 150K. The question I am struggling with now is to do a stock rebuild of the diff or upgrade to 4.1 gearing. I'm finding an additional $1100 on top of the $1400 rebuild hard to justify, but seems folks are very happy with the 4.1 for split track/weekend duty which is my situation.

Never driven 4.10 but seems like a no-brainer for effective power increase to the wheels. May find yourself needing to shift more. At Summit Point (my home track) with the stock gearing I am basically at the very top of 4th at the end of the main straight. The extra shift may actually end up with a slower lap time, but the extra power would likely be more fun to drive. I would call up diffsonline and just talk with him about the build before ordering. He is pretty friendly and knows a ton about these diffs.

Curious, what exactly was "shot"? And why does the driveshaft need to be replaced?

[heinzboehmer]

There's also the option of going with 3.91 gears, if you want a bit more torque but still want to be able to drive around town without shifting five million times a minute.

[ATB88]

There's also the option of going with 3.91 gears, if you want a bit more torque but still want to be able to drive around town without shifting five million times a minute.

I heard those went NLA because they were discontinued last year and replaced with 3.86. Have they become available again?

[heinzboehmer]

I heard those went NLA because they were discontinued last year and replaced with 3.86. Have they become available again?

The diffsonline website doesn't say anything about them being no longer available, but it's entirely possible that it's out of date.

https://diffsonline.com/bmw-e46-m3-differentials.html