RS2 high frequency rear diff vibration

[Avantus]

Hi guys,

Working on an RS2 that has a high frequency vibration on the drivetrain. Only starting around 50mph, and then frequency gets gradually higher the faster you drive.
Acceleration, steady, or decelleration doesn't matter... it's there.
So it must be something in the gearbox/prop/diff area as it really doesn't feel like a wheel speed vibration, its faster pitched than that.
When you sit in the rear you really feel it in your feet, and the drone/vibration can be heard in the boot.

What has been done:

-propshaft rebuild with new U-joint, CV-joints, center bearing from GT500, and complete unit balanced to under 5gr
- made a bracket to mount the prop perfectly straight. After that even played with different shim height to rule that out. Also aligned the diff straight with prop.
-rear diff mount filled with window bondo, and also the rear diff carrier bushes.
-all wheels perfectly straight and balanced. Swapped them with another good set just to be sure, no difference.
-half-shafts in the rear new boots with fresh grease.

As a final test we mounted a different propshaft yesterday from another RS2 that drives without vibration -> problem stayed identical!

The diff has some play, but it doesn't howl or that sort of noise. Also can't notice any height play in the input shaft.
We dropped the diff oil, and didn't have any noticeable metal in it, BUT there was only 0.5l in the diff. So that's not healthy for sure.

Has anyone encountered such things?

Any input is welcome

Cheers!

[Vauhtiveikko]

How about wheel bearings? Wheels off the ground and one hand on the suspension spring while the other hand turns the wheel. Any vibration?

[Bowie69]

Certainly sounds like a wheel bearing. Other option is diff carrier bearings, but to check that it is driveshafts off.

Does the pitch change when steering left/right?

[Avantus]

Wheel bearing LR is new, and RR has no play.

Forgot to mention we drove the propshaft before it was balanced (in the hope it would be still ok). Vibrationwas then between 40-65mph... beyond that it was actually OK.
But I'd rule the propshaft out because it doesn't response to playing with height, and another one didn't make the slightest difference.

thanks guys!

[Bowie69]

I would put the propshaft alignment back to standard, or as close as possible, running mismatched uj angles can cause noise.

Could it be gearbox output bearing? It is quite far back, near rear seat passengers feet.

[Avantus]

Just noticed that the output shafts on the diff have some vertical play (i can knock it up and down 2mm or so)... is that allowed or should it be rock solid?

[Avantus]

Glad to report that the vibration is completely gone with re-filling the diff (1.3l). Hope it remains ok, but it doesnt make any noise and car feels smooth again.

Thanks for the help!

[piotr]

Good news