Constant Loud Drivetrain Whirring Noise While Moving

[bmwman91]

I noticed last night that I get a pretty good amount of constant whirring from the car when I am driving. It is most apparent when I am coasting in neutral since the engine isn't making as much noise. There is some speed dependence to how loud it is and as I get down under ~5MPH it goes away, but there is no vibration or anything to indicate something is out of balance since it is a constant whirring sound. It isn't a brake caliper being seized since the wheels seem to spin freely and I can coast just fine.

It seems to have gotten worse over the last couple of months. I have a 4.10 LSD (small case) in there with a lot of miles on it...are there known failure modes for those? Are there any good ways to test the diff and tranny in the garage to see if there is an abnormal amount of rotating resistance/friction? I am suspecting that something may be dragging somewhere in the driveline which is also resulting in at least part of my poor gas mileage. Yesterday I averaged an actual 26.5MPG (OBC read 33.6MPG...but I have a custom engine and different injectors) on a 250 mile drive where I behaved and averaged 66MPH, which seems pretty low even for a built 2.1L M42. My mileage has actually been declining steadily over the last 5 years or so, and despite replacing everything in the injection system, nothing has really made a difference.

I would try to record a video to capture the sound, but I think even though it is quite loud, it wouldn't come out very well.

[DesktopDave]

I'd suspect the drive shaft or potentially a rear wheel bearing. You didn't mention if it gets worse while turning.

Check your CSB, if you haven't already. I also had a small case diff go bad (input bearing), but it was a horrific 'wub wub wub' sort of bearing noise. Low pitched, not anything like the higher pitched whine of a bad CSB. The medium case 3.73 I replaced it with was an excellent choice IMHO, although I'm not sure what would fit best with your hot-rodded MM motor.

Could also be the guibo or u-joint, but those rarely go wrong in my (admittedly limited) experience.

[bmwman91]

Thanks. The noise is hard to describe...almost a mix of a whir and grinding. Definitely makes me suspect a bearing or joint of some sort. The CSB is only 18 months old so I would imagine it is OK. The rear wheel bearings have never been replaced as far as I know, and the LSD was a forum-find 7 years ago so who knows what shape it is really in. Sounds like I will need to dig in this weekend and start checking things more closely!

The noise does get a little louder as I go, but at highway speeds the road noise swamps it out. It is most noticeable under 40MPH.

[Tgoode318]

I was going to say check your csb and guibo, but then you say loss of Mpg and now some metallic noise's. That makes me think it's your diff, I would try a full flush and go to redline or royal Purple gear oil and see if things quiet down; at the very least your mpg should improve. While your down their inspect your diff mounts and make sure your guibo isn't cracked. Something that made me feel (some what) better was seeing all of the 2014 owners having problems with their disc after only 10-15k miles   so us e30/e36ers  that have over 50-200k should feel lucky!  I know I am grasping for straws here but ill embrace any philosophy that makes me feel better about spending money on over priced rubber

[locknload]

Before you start buying/swapping out parts, I'd recommend just putting the rear up on jacks and start turning the rear wheels by hand; one side, then the other while paying close attention to the sounds & feel of the moving parts.  Try this both in & out of gear, but you will need someone to hold the clutch pedal.  Try all the permutations: in gear, out of gear; forward, reverse; etc.  You might also try driving along a concrete/brick wall in a quiet location & listen with the window down.  Also helps to have a knowledgeable person in the back seat listening as well.  See if you can equate the frequency of the noise with the various rotational speeds of the different drivetrain parts.

[deansweet]

Before you start buying/swapping out parts, I'd recommend just putting the rear up on jacks and start turning the rear wheels by hand; one side, then the other while paying close attention to the sounds & feel of the moving parts.  Try this both in & out of gear, but you will need someone to hold the clutch pedal.  Try all the permutations: in gear, out of gear; forward, reverse; etc.  You might also try driving along a concrete/brick wall in a quiet location & listen with the window down.  Also helps to have a knowledgeable person in the back seat listening as well.  See if you can equate the frequency of the noise with the various rotational speeds of the different drivetrain parts.

x2.

Try to wiggle the back wheels while off the ground as well. That will help determine if the bearing is failing.
I am guessing it's one of your half shafts. Then tend to make metal type noise and make a weird whirring noise when they start to wear.

[Gabs]

I've had something similar years ago, that ended up being a rear wheel bearing. Though is was less of a whirrr, more of a vibrating drone. Are you getting any vibration in the cabin? For me it was worst at about 35-40mph and I believe partially went away when I turned left. It was felt most up the center of vehicle, or at least that was my perception of it. Noises like this are hard to pinpoint.

[Tgoode318]

I have replaced multiple shafts but never had a significant loss in mpg. A bad wheel bearing is a good idea, it can cause metallic noises as well as a loss in mpg from parasitic drag i would inspect yours along with doing what dean said, +1 to gabs.

[locknload]

Any resolution on this?  I'm starting to hear the same kind of noise on my 4dr.  Would love to hear it's not a diff issue.