Mystery "whirring" at 40mph

For the last month or so, I have noticed a higher-pitched whirring noise coming from my car that I can't seem to diagnose. The noise is faint and can only be heard from 40 to 45mph and it only occurs when I am on the throttle. If I let off the throttle, the noise goes away. I have produced the same sound in 4th, 5th, and 6th gear (6MT), so I have ruled out a transmission-related issue. Once I accelerate beyond 45mph, the sound goes away.

Has anyone else had a similar issue? I have searched the forum multiple times and haven't been able to find a potential fix... Any help is greatly appreciated!

Is it coming from the front or rear? Or a particular wheel?

If rear might be differential whine.

Redd wrote

Is it coming from the front or rear? Or a particular wheel?

If rear might be differential whine.

Honestly it's hard to tell. the sound seems to move around the cabin. I'll try to listen again on my way home and see if I can determine.

Speed specific regardless of gears or rpm points to either wheel bearings or rear differential. Wheel bearing hum tends to get louder as speed increases, so I'm guessing rear diff.

I took the car out last night after traffic cleared up and have confirmed it's coming from the rear. I also confirmed hub temps are the same left to right, so I'm assuming it is diff related. I will go through my service records from the previous owner, but I don't recall seeing a diff flush. I just hit 62k, so it's probably long overdue (though I recall reading in the manual that the diff doesn't need service?).

Bearings can wear out even at 62k. Or it might just be low on fluid or need some friction modifier. A good workshop will be able to tell you what needs doing.

OK, so I had my shop complete the diff flush, and it was apparent the diff has never been serviced. I am glad to have that done; however, the sound still remains.

My mechanic inspected the rear bearings while it was on the lift and didn't find any additional slop that indicates a bearing failure. That said, we took the car for a spin and he seems to think the left wheel bearing could be going bad, due to the location of the whirring within the cabin.

I am going to replace both rear wheel bearings for peace of mind and hope that does the trick (if not, I still have new bearings that won't need replacing soon). I am on FCC Euro's website and it appears many use the F.A.G. OE bearings P/N #33416775842.

From what I can tell, they are the same bearing as the BMW OEM unit but at a lower price. Does anyone have experience with these?

NVM, I was able to go through a stickied post about OE parts and have confirmed that P/N is the same as the BMW-labeled part. Will be ordering today and will check back with an update once installed!

jmeenach wrote

NVM, I was able to go through a stickied post about OE parts and have confirmed that P/N is the same as the BMW-labeled part. Will be ordering today and will check back with an update once installed!

update?

parkjake50 wrote

update?

Nothing to share yet. I've replaced both front hubs (one of which was bad), serviced my diff, and ordered rear wheel bearings that will go in next week.

The car decided to eat a chunk of pavement that was shot up by the car in front of me, which ripped out my skid plate, oil pan, one of my primary cats, and some other goodies underneath.... so the car is currently undergoing some major repairs that have slowed me trying to chase this issue. That said, it may be a good time for me to disassemble the rear diff and see if there is a bearing going bad there as well. ATM I'm just thankful there was no damage to the motor internals!!

As it relates to the P/N for the front hubs, the additional brake dust on the front right is now gone and I've had no issues. Would recommend if you are in the market for new front hub.

Circling back on this. I've installed new rear wheel bearings, guibo, CSB, and completed diff services with FM fluid. I still have the same sound. I wonder if the sound is coming from the driveshaft u-joint...

Multiple shops have said there is no play in the driveshaft, and the sound hasn't become louder, but I'd still like to find the culprit a) for peace of mind, and b) to make sure I don't have a catastrophic failure while on track.

Any ideas here?

jmeenach wrote

Circling back on this. I've installed new rear wheel bearings, guibo, CSB, and completed diff services with FM fluid. I still have the same sound. I wonder if the sound is coming from the driveshaft u-joint...

Multiple shops have said there is no play in the driveshaft, and the sound hasn't become louder, but I'd still like to find the culprit a) for peace of mind, and b) to make sure I don't have a catastrophic failure while on track.

Any ideas here?

Post a recording of the sound here for better help and analysis.

Hey I have a 2011 M3 that I think is making the same noise…anything above 47mph in a straight line while accelerating or decelerating and the noise gets louder with speed. Been doing research on these forums and haven’t found a exact fix yet…Seeing everything from bad diff bearings to low power steering fluid to bad DCT mechatronic seal. I have no record of the rear diff or trans service so will start there and let you know if that fixed any issues.