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      06-27-2016, 10:33 AM   #1
wss4
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Differential noise/rumble - high speed off throttle

I installed AKG's softest poly bushings in the subframe and differential about 10k miles ago. After installing, I had a light gear whine at higher speeds off throttle, which I enjoyed and deemed normal with the stiffer bushings. More recently, the car has developed a very loud and aggressive rumble/vibration from the rear end when I let off the throttle at higher speed (90-100 mph). As speed drops below 90, the rumble transitions back to the whine I am used to, and the aggressive vibration goes away. Above 100 mph, the noise and vibration is also subdued when I let off throttle (versus 90-100 mph).

Car (Z4 M coupe) has 80k miles on it. Acceleration is smooth and quiet throughout all speeds. Mount bolts for subframe and differential are tight. I am suspecting something internal to the differential (bearings or preload?) that is being made obvious by the stiffer bushings, but do not have much experience with the guts of differentials. Any help, recommendations, or knowledge on the issue would be very much appreciated!
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      04-10-2019, 03:36 AM   #2
MBan
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Sorry to revive an old thread but did you ever resolve this issue?

My car whines in the same way you’ve described for yours but at a lower speed (60-70mph) and I don’t have the rumble you describe.

All my bushes are factory original.

My garage has a couple of theories but it would be good to hear first hand from someone who has fixed the issue.
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      04-10-2019, 10:21 PM   #3
wss4
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I had a combination of issues. My stock differential was worn out by the 70-80k mile mark. The factory differential bushings were dead well before that point, and I believe the stiffer AKG bushings solved the clunking under hard acceleration, but then made the other wear in the differential much more audible. The lash, clunking, and whining was for the most part eliminated after I put in a new differential. I was even having difficulty shifting/engaging gears, and this was significantly improved after the new differential (much to my surprise).

The rumbling, I suspect, was due to a failing axle CV joint. I had the rear wheel bearings replaced at ~60k miles by a well known shop. I discovered much later that the technician had hammered the axle back into the hub (deep hammer marks on the CV joint flange). I experienced clicking from rear driver side wheel on and off for almost 30k miles. It would click under certain drivetrain and suspension loads for a few days, then go away altogether for 10k miles. Finally the clicking returned and did not go away until I replaced the axles.
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      04-11-2019, 04:37 AM   #4
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Thanks for replying; it sounds like you had a bunch of things going on at once but i’m glad you sorted it all out.

Did you put in a new standard differential or go aftermarket?
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      04-16-2019, 07:21 PM   #5
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I also had grumbling at high speed decel. One cause was the type of tires I run (R888R), but when swapping to other tires, I realized for sure it was the diff.

When you decelerate (or coast), the pinion and ring will move away or toward each other. In my case, the pinion was slowly developing more in-out slop. You can test this by doing a force test on the pinion, rotating the pinion flange with an inch-lb wrench, and measuring the breaking force.

Mine eventually had none, to the point where I just snapped a tooth off the pinion gear during recent HPDE event.

You may need to remove the diff and just get it serviced with new compression sleeve for the pinion.
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      04-18-2019, 06:06 AM   #6
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Thanks - that’s a really good explanation.

I’ll get it back in to the workshop in a couple of months and see what they can find.
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      04-18-2019, 05:41 PM   #7
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Quote:
Originally Posted by dre99gsx View Post

When you decelerate (or coast), the pinion and ring will move away or toward each other. In my case, the pinion was slowly developing more in-out slop. You can test this by doing a force test on the pinion, rotating the pinion flange with an inch-lb wrench, and measuring the breaking force.
But this would be bearing slop right?
Slop between inner and outer bearing on the pinion shaft should be 0 absolutely and should even be under pretension. (that you also measure with an inch-lb wrench but not the click type but the bend type, as you try to measure drag). But this measurement should be done with only the pinion installed in the housing, not with core and crownwheel installed.

Quote:
Mine eventually had none, to the point where I just snapped a tooth off the pinion gear during recent HPDE event.
Lol been there done that:

(I had some issues with my quaife atb that was previously unknow at quaife's but funny enough the quaife core survived all this! )
but this is a 188k diff though, not the type 215.


Quote:
You may need to remove the diff and just get it serviced with new compression sleeve for the pinion.
A new compression sleeve is only necessary if new pinion shaft bearings are fitted or if it has been overtightened.


Quote:
Originally Posted by MBan View Post
I’ll get it back in to the workshop in a couple of months and see what they can find.
Be sure that if you have a 'shop' opening up your diff and have it disassembled, that that shop is more a transmission/drivetrain specialist than a 'shop'
Adjusting a diff and and taking the measurements how they should be taken is very much a specialist's job and is usually a bridge too far for a normal mechanic.
Also have them take a look at your propshaft (centerbearing, weld on balance weights, giubo and cv joint (diffside)).
Take into account that you have the 'softest akg subframe bushings' but at shore 95a they are just as hard as most other polybush suppliers.
So as far as aftermarket subframe bushings go, these are stiff (so noise).
Doesnt mean that you now probably have a 2nd problem.
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      05-03-2019, 12:39 AM   #8
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Thanks, Guido.

I’ll see if I can find a transmission specialist with BMW M experience.

All my bushings are completely OE, unless you’re referring to wss4?
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      05-03-2019, 05:04 AM   #9
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Quote:
Originally Posted by MBan View Post
All my bushings are completely OE, unless you’re referring to wss4?
Sorry, yes I got things mixed in my reply.
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      08-19-2020, 09:16 AM   #10
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GuidoK - when I rebuilt my diff, I did preload test on pinion only. I've been suffering from a clunk when i engage the clutch going reverse or forward from a standstill. Odd thing is my Supra has the same clunk from factory (us Supra guys have an excuse that heavy duty drivetrain on Supra needs some slop which is why we get clunk).

I have OEM guibo, oem driveshaft, but the red poly bushings for the rear diff. When I went to investigate the clunk originally, I noticed I melted off the rear diff bushing and could move the diff up and down with my hand! Well, after replacing new bushing, it's in there solid, but clunk remains. More to the story...

After I broke the rear pinion (mind you, I don't launch the car, strictly HPDE driving), I rebuilt it by just replacing with another 60k mile 3.0si 3.62 auto diff, just swapped in my Giken LSD, swapped diff bearings and reshimmed the diff, and hit the pinion nut a bit for some more preload. I was measuring 0 preload on the pinion with no breakaway torque. Suspect...

Ran on that setup for a few more events, still had decel grumble. After a few more events, I decided to rebuild it again. This time, all new bearings throughout, new pinion crush to 20"lb preload (pinion only), .025" backlash on pinion/diff gear, and Motul 75-145.

Thump continued. Decel rumble continued. 3 HPDE events on it now. If I break another tooth I feel it could be the power limit of this diff (remember, 520ft/lb to the wheels on the 2jz swap...). Combined with heat from these events and no active diff cooling, who knows.

I don't think the drive shafts are causing the decel rumble, it wouldn't make sense. Happens in any gear and is speed dependent. In other words, 80mph decel in 5th gear or 4th gear, same rumble. I did replace the driveshaft and new guibo, no change.

Not sure if upgrading to Z4M diff (and subframe and all that jazz) will help me much, they are the same pinion/ring size if I'm not mistaken (E46 330i diff vs E46 M3 diff same comparison)
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      07-01-2022, 12:19 AM   #11
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I know this isn't my thread, but I thought I would provide an update on my experience. A few weeks ago I had new suspension installed (KW V3) and at the same time replaced a bunch of bushings, including the diff bushings. I also replaced the diff bolt with the 'upgraded' version. And now, the whine is gone. Apparently all the bushings on the car were in very good condition (it has low kms), so maybe it's the bolt that helped? Or maybe just re-tightening everything to install the bushings was the key? Who knows, but I was pleasantly surprised when I noticed the whine had gone.
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