Need help with some high speed vibes

[Bobs]

I have been noticing some sort of vibration/hum coming from what I believe to be the back half of the car when driving at speeds in excess of 75mph. It almost sounds like a subwoofer playing a single note that increases in volume with speed. Dumping the clutch on the highway doesn't change the sound in any way.

Looking for an easy fix I had the tires rebalanced but it didn't solve the issue. Any suggestions on where to look next?

[Tbonem3]

Wheel bearings?

[eacmen]

Wheel bearings?

That's the first thing that I thought of too.


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[Speed Monkey]

After the wheel bearings have been checked, and if that doesn’t resolve the noise, the peanut gallery 😁 suggests the following be looked into as a possible solution...

Some background first - some years ago on a rainy day that left a fair amount of standing water on the road surface, I drove into a massive pothole that was fully concealed by it being filled with water. The impact was so violent that It sounded like a go##amn explosion went-off in my car; it also scared the tar out of me.

Anyways, soon after, at highway speed, my sled experienced something along the lines of that titled in the OP’s post. At my wit’s end and last desperation, I took my sled to my wheel guy. He put my front wheel on a Hunter Road Force balancing machine and it revealed the pothole impact made my wheel slightly oval in shape; it was not discernible to the eye that the inside barrel was ever so slightly oval. I took it to HRE, got it re-barreled, and voila! Happy motoring again.

[Bobs]

Per everyone's advice I finally got around to changing both rear wheel bearings. I went ahead and replaced RTABs, FCABs, swapped the diff oil and repacked/sealed the inner CV joints on both half shafts while I was in there. It wasn't fun, but I got it done.

Unfortunately it didn't fix the vibes/hum whatsoever. Any thoughts on what to look at next? Maybe something related to the driveshaft such as the CSB or rear u-joint? Do either of those cause that sort of sound when they're on their way out?

[Mattn1192]

Per everyone's advice I finally got around to changing both rear wheel bearings. I went ahead and replaced RTABs, FCABs, swapped the diff oil and repacked/sealed the inner CV joints on both half shafts while I was in there. It wasn't fun, but I got it done.

Unfortunately it didn't fix the vibes/hum whatsoever. Any thoughts on what to look at next? Maybe something related to the driveshaft such as the CSB or rear u-joint? Do either of those cause that sort of sound when they're on their way out?

Notice anything different (in a positive way) when u changed ur diff fluid? I ask only because I plan on changing it this weekend.

[Bobs]

Notice anything different (in a positive way) when u changed ur diff fluid? I ask only because I plan on changing it this weekend.

Maybe a touch more quiet, but I've only gone on one drive since changing it and was more focused on the other issue.

[Mattn1192]

Which fluid did u go with

[Bobs]

I went with the BMW 75W140 SAF XJ+FM as I was in a rush. If I had more time I probably would have gone with the Castrol Syntrax LSD and added some friction modifier myself.

[EthanolTurbo]

Per everyone's advice I finally got around to changing both rear wheel bearings. I went ahead and replaced RTABs, FCABs, swapped the diff oil and repacked/sealed the inner CV joints on both half shafts while I was in there. It wasn't fun, but I got it done.

Unfortunately it didn't fix the vibes/hum whatsoever. Any thoughts on what to look at next? Maybe something related to the driveshaft such as the CSB or rear u-joint? Do either of those cause that sort of sound when they're on their way out?

Try the Road Force balancing another poster suggested above. It's crucial for BMWs.

[jayjpl]

I have been noticing some sort of vibration/hum coming from what I believe to be the back half of the car when driving at speeds in excess of 75mph. It almost sounds like a subwoofer playing a single note that increases in volume with speed. Dumping the clutch on the highway doesn't change the sound in any way.

Looking for an easy fix I had the tires rebalanced but it didn't solve the issue. Any suggestions on where to look next?

https://nam3forum.com/forums/forum/main-forum/e46-2001-2006/100278-weird-noise-vibration-in-cabin-at-high-speeds-solved

I had a similar issue to yours, read this and let me know if these sound similar.

[Mattn1192]

I went with the BMW 75W140 SAF XJ+FM as I was in a rush. If I had more time I probably would have gone with the Castrol Syntrax LSD and added some friction modifier myself.

I was under the impression that the oem BMW diff fluid was the best one and already had friction modifier in it.

[enjoy_m3]

Put the car on a lift, and run the car in gear, maybe 3rd or 4th and see where the sound is coming from. Only way to find out. Check the center bearing. If sound is coming from the diff, your pinion bearing is likely toast.