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Discussion Starter #1 (Edited)
Occasionally while cruising around 45 - 60 mph a noise will start. It's loud enough to hear over the radio and is usually only there for 30 - 60 seconds. It's not rpm related (I push the clutch and drop to idle, the pitch doesn't change), not the radio (turned off or on, doesn't matter), not the AC (again, off or no and no difference), and I'm pretty sure it's not the automatic climate control fan.

It almost sounds like I'm driving over grooved pavement, just a constant raarrrrrrrrrrrrrrrrrrrrrrr.... sound. That's the best I can describe it. I know it's coming from the front, but not if it's left / center / or right side. At first I thought it was coming from behind the gauge cluster, but I don't know at this point.

My guesses so far -
* Wheel bearing
* Pilot bearing
* Drive shaft bearing
* ???

I believe the clutch is still the original (98K miles). Power steering fluid is at the correct level and it's very intermittent. I hear it maybe 4 - 5 times a week, usually never more than once during a drive.

Ideas?
 

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I have a subtle noise maybe similar to this that happens when I get over about 80mph. Opening/closing the sun roof changes the pitch. So does switching the air recirculate off/on. Mine you can hear very well over the radio unless it's talk radio on low. Haven't really spent much time troubleshooting it as I've other things to focus on.

Wheel bearing should be pretty constant, not intermittent once it starts to fail.
Pilot bearing noise should go away when the car is in gear and clutch engaged as the input shaft and FW should be spinning in unison.

On one of my other M5's (not the car above) the motor mount was bad and made a noise like a bad wheel bearing. Disengaging the clutch but coasting the noise would stop when the load was off the drive train.
 

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M5 Paranoia? I hear "dead people" noises all the time when I drive mine :)
 

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Discussion Starter #4
I located the cause of the noise tonight! Sunroof in the vented position, windows up, AC on auto, but was running a low fan speed and blowing on the floor. The noise started at 40 - 45 mph, got on the brakes hard and the noise faded to a stop. Sped back up and it started again. Changed the AC output to blow out the windshield defroster and the noise instantly stopped.

So it has to do with air being pulled out of the car around 40 to 60 mph. Sounds like a flap or something in the AC reverberating. Anyone ever seen this before?
 

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Discussion Starter #7
It's 100% something with AC system.

Heard the noise today, cycled through Auto, Recirculate, and Manual (Open I'm assuming). On Auto and Recirculate, the noise was prevalent, but the second I turned it to Manual (Open), the noise stopped completely.

Any ideas what or how to fix this?
 

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It's 100% something with AC system.

Heard the noise today, cycled through Auto, Recirculate, and Manual (Open I'm assuming). On Auto and Recirculate, the noise was prevalent, but the second I turned it to Manual (Open), the noise stopped completely.

Any ideas what or how to fix this?
It looks like in Manual mode as you call it, the fresh air valve (which I think is the same as the ram air compensation valve referred to in the PDF) is open to varying degrees depending on car speed and all the air coming in is from the outside. In AUC or Recirc mode, the fresh air valve is closed and the Recirc valve is opened so only air from inside the car is cycled around. (In AUC mode, the two valves are cycled open and closed depending on the outside air quality, assuming your AUC sensor is still working.)

Since you hear the noise in AUC and Recirc only, does that mean it's related to the Recirc valve being open, or the Fresh Air valve being closed? The flaps on these valves are usually sealed to the duct with flexible rubber so I could see it being some of the rubber coming loose. Or maybe some debris has got sucked in and is blowing about or holding the fresh air valve open a bit.

The fresh air valve and the recirc valve have to be in the ducting on the suction of the blower. That means they are at the front of the IHKE assembly near the firewall, which is bad news as they will be tough to access, IMO. Sorry, I don't know what to suggest, except BMW has been using a similar design of IHKE assembly for years so maybe a dealer or indy has seen this before.
 

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You may want to also check your aux fan. It's not on all the time, but changing the a/c settings may trigger it. If something is rubbing in there, or the bearings are going it could make a rumbling noise.
Ed
 
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