[vandentr]

I have a vibration/hum that is always present, but when I hit 40mph the harmonic is so loud that it's almost a deafening roar! It's low enough in frequency to be impossible to locate from inside the car. So I can't tell if it's coming from the front, middle, or back, and the kicker is that it's ONLY dependent on speed. It doesn't matter if I'm in neutral and coasting, or under load or accelerating or decelerating or turning.

I'd think if it were a torque converter, axle, wheel bearing, or drive shaft related there would be a tonal difference in the sound when under varying loads, or turning, etc. There is none of that, it's 100% steady depending on speed.

To begin the diagnosis process, this past weekend I pulled the exhaust and heat shield and inspected the drive shafts, GUIBO, and CSB, but all look good and feel tight and solid.

:banghead:

The only noticeable issue from this inspection is about 2-3mm of lateral/axial movement (not rotational lash) on the front output shaft of the Transfer Case.

Could this be the culprit?

 

[Kubica]

Did you try different tires?

 

[vandentr]

The tires AND Wheels are less than 6 months old, but the hum has been going on since before those were purchased.

:hmm:

 

[GuiltySpark]

Transfer case maybe?


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[r111093]

Definitely not the safeast thing to do at all but my daredevil self would put the car on a lift and get it to 40mph and then try to locate the noise...

 

[vandentr]

Well, my daredevil self put it on jack stands and attempted that very thing!

However, the car's too smart for both of us. The traction control refuses to cooperate and barely lets the the tires start to spin before cutting the power from the tranny.

I'm gonna run a plastic tube "extension" attached to a stethoscope to various parts of the car underneath to try and isolate the source. Then I'll run the tube up through the window and listen as I drive.

I honestly don't know what else to do since it appears to be an extremely rare situation, which seems to be a consistent theme in my life!

 

[Gunsnbacon]

See if a local shop has a rolling track/dyno that's either raised up or has space under it so you can get it up to speed and look.

I have the same movement on my output shaft at 200k miles and it doesn't make any noise.

Maybe try to isolate the problem from the engine and drive train. Run it in neutral at various revs with a large work fan blowing in the front of your car to prevent over heating. At least this will rule out the vanos system rattling/buzzing... This can come up at about 3000-5000 rpm.

 

[vandentr]

I have shut the engine off while coasting in neutral and the vibration persists in exactly the same manner as when it is in drive - neither the engine nor the degree of load makes any difference.

:ben:

Based on everything I've done so far it seems like it would have to be somewhere between the input shaft of the Transfer Case and the input shafts of the front/rear differentials.

I wish I could hear what a failing GUIBO sounds like first hand...

 

[SamDoe1]

Why don't you turn the traction control off and then do your jackstand test? BTW, you have balls for doing that. :rofl:

If the TCS off button doesn't do anything, take out the fuse for the whole system to disable it. Make sure you put it back though... DISCLAIMER: Not sure if this will throw any lights that you'll need a stealership to turn off, do at your own risk.

I don't think a failing guibo sounds like anything, just a vibration. Have you checked all the wheels for anything bent or rubbing? Have you driven with the belly pans off of the car? Check the exhaust heat shields and make sure they aren't hitting anything and are secured down. Check the rear diff mount and all suspension mounts. Have you lowered the front subframe recently? If so, did you replace the bolts?

 

[vandentr]

I did try to disable the traction control with the button, no dice. BMW apparently doesn't screw around when it comes to protecting the driver and AWD system on these cars. I will look into the fuse option, but I'd hate to create a bigger problem with the tranny, that makes me even more nervous than revving the engine with the car in gear while on jack stands!

1) The wheels and tires are pristine, and there's no shimmy in the steering wheel or shudder in the cab.
2) This is a deep, low, and powerful vibration, there's no way a heat shield could cause what I'm experiencing.
3) There is no clunking that would lead me to think that a bushing or mount would be the culprit.

Remember, it's speed dependent - and the slower I go, the lower in frequency it gets. It's ALWAYS present, but it isn't until about 30mph that it rises above the 20Hz audible range the human ear can hear. At 40mph for an extended period of time it becomes almost painful, at 50 mph you can barely tell it's there, at 80mph it's higher and about half as loud as at 40mph.

I could try to record it, but I'm afraid it's too low for the recorder to pick up on. I'll give it a try though, what've I got to lose?

My other thought was to remove the front drive shaft and have a friend tow me up to 40mph and see if the problem goes away, if not, then do the same with the rear shaft. It'd be a pain in the ass from a time perspective but I'm at a loss for other ideas. The bottom line is that I'm not throwing parts at it, I'm going to locate the source first no matter what.

Thanks for all the input by the way, it's encouraging me to keep picking away at it!

-TVB

 

[Gunsnbacon]

Wheel bearings can hum at various speeds but they don't sound that loud and usually sound a bit like grinding.

 

[Bgilby]

I am having the very same issue at around 35-40 mph. My car is at the dealer today for regular service, I'm hoping they identify the issue.

 

[SamDoe1]

It might be time to take this thing into a shop before you start doing anything that extreme. $100 to find out the problem would be worth the money rather than wasting so much more of your time chasing it.

 

[vandentr]

I know Sam, but to me that's like admitting defeat.

Also, I've read countless stories where the shop says, "We don't know" to issues like this involving the XI drive train and then proceed to start guessing and throwing parts at it. It's just a pain in the butt to diagnose due to it's design.

I need to get to the bottom of it because I couldn't even sell it like this with such an obnoxious noise, it's obvious that something's wrong.

I'm curious about what the shop discovers on Gilby's car, hopefully he updates us with the results. Mine can't be the ONLY one of the 325xi fleet that's run into this issue, can it?!

 

[SamDoe1]

It's not admitting defeat, it's asking for help. Either way, just have them diagnose and then fix it yourself.

Have you given a good look at the wheel bearings though? Even though the sound isn't direction dependent, the fact that it's a constant drone points me to tires, bearings, and other rotating things. You're right that the drive shafts and such would change in sound as they load up/speed up so I would discount those as well.

Other thing to check out is wheel balance.

 

[vandentr]

I can say with 100% certainty that it's not the tires.

When I bought the car last summer the noise was there, just not as pronounced, but there was also a wicked shimmy, which turned out to be 2 bent rims. I said fu*k it and bought 4 new rims and tires. The car rides like it's on glass, nothing in the steering wheel, doesn't pull, it's tight like it should be.

Every experience I've had with bad wheel bearings is that when you "lean" the car into the bad one it really complains. I can do figure 8's with mine and there's no change to the pitch or frequency of the hum unless I change the speed.

It is a profound mystery, and as such I'm sure I'll be dragged, kicking and screaming, to the shop before it's all said and done...

:read:

 

[SamDoe1]

When's the last time you changed out the drivetrain fluids?

I agree on the wheel bearing symptoms but it's worth a listen. Just take the wheels off and use a stethoscope to see if anything sounds out of whack.

Wait, so you changed the tires/rims and it got worse?

 

[GuiltySpark]

I am having the very same issue at around 35-40 mph. My car is at the dealer today for regular service, I'm hoping they identify the issue.

Mine is at 40-45ish. The dealer said it was the tx case humming like that. It's not particularly noticeable but I drive the car 24/7 so I notice the small things.


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[Bgilby]

no surprise, dealer says my sound is normal ... maybe a little tire noise. I don't agree.

 

[Bgilby]

I was back at the dealer today for a bluetooth problem and asked them to check the vibration noise again because it is really loud. Well it turns out the Continental ContiProContact SSR tires that were on the car when I purchased are all shot to hell. 16,000 miles total on the car now, 5,000 from me, and the front tires are crap. I can feel the uneven wear with my hands. I'm a little pissed that I'm barely 5 months into a new car and now I need to buy new tires.