Troubleshooting diff noise, 3.15 replaced with 3.64
 

Hey everyone,
I just spent my Saturday replacing my 3.15 with a 3.64 in my 330i. I've swapped out the fluid and output shaft seals to make sure its completely sealed. After installing the refreshed diff and going for a drive the first thing I noticed was a whirring noise when letting off the throttle at low speeds. When accelerating or driving at higher speeds (so in third gear and onwards at this point) the noise isn't present.

So putting on my thinking cap I've jacked the car back up and inspected it further. While under the car I've noticed some backlash (play) from the driveshaft connecting to the input flange to the diff and when rotating the driveshaft it reproduced that same noise.

My question, with the whirring noise and the backlash present do you think its the bearings in the input side that are bad or is it something more? Also, is it dangerous to drive with this? I have a trip coming up next weekend and debating swapping my old diff back in.

Limited slip?

Neg, just upped the gearing

hmm, I'm running 3.73 lim. slip on my BMW and there's some light whirring sounds at low speeds that's commonly been considered "normal"

Can you specify exactly what was replaced? Did you swap the guts in your diff or replace the diff? If you swapped the guts what exactly did you install/replace?

Judging by the symptoms I'm going to suggest that there is excessive clearance somewhere... sounds like a backlash issue.

Bought an entire 3.64 diff, replaced the two seals on the output shafts. Everything else left as it is.

Is the drive shaft two piece?

If so, was it lined up again as it was installed from OEM? If its off by more than a few splines it could cause noise or other vibration issues. There should be some markings from OEM as it was balanced.