4:10 diff- noise

[e90M3joe]

Hello-
Last year I purchased a 4:10 diff (complete-not just gears) for my 09 e90 6sp M3. Ever since the install, i notice an audible, hi pitched whine from about 5k to 6.5k RPM. There is no discernable whine prior to, or after that RPM range. It is somewhat annoying. I was told this is normal due to the fact that the ring and pinion are from BMW Motorsport. Thus, the gears are "cut" differently? While I don't doubt the source, I just wanted to confirm this with the group and to see if any other members with 4:10 diffs have a similar experience...

Other than that, the diff works fine (more torque!) and I'm using the the stock diff mounts.
Im using Redline diff fluid- 75-140 with 1/2 bottle (4oz) of friction modifier. Would switching to the BMW fluid be beneficial...? thanks-

Joe

[Beemdog]

Member driftlo (on this forum) has a lot of experience setting up differentials (see big LSD shootout thread). Might PM him for an opinion on your issue.

Personally, I had a 12-bolt Chevy rear-end built with a set of 3.73 gears and it whined at first due to improper setting of the pinion depth (into the ring gear). However, it is true that certain gear cuts do tend to whine, not sure about the BMW Motorsports offerings.

[DR-JEKL]

there are quite a few threads on diff fluid, i know i read one guy had diff whine with redline dropped it and added bmw fluid and no more whine.

Of course it may not be as simple as that, but diff fluid is cheap so perhaps try that?

[thekurgan]

Sounds like a preload issue. NO fluid is going to change the sound. Do you hear it at partial throttle or does it change from on/off throttle?

Half a bottle of the fm is far too much. You should only be adding enough to eliminate chatter or a bit during break in.

[FogCityM3]

Who is the diff manufacturer?