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      01-16-2021, 10:55 PM   #1
kroymander
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N55 high-pitched electrical noise from alternator?



2011 135i with ~90k miles. Stock except for charge pipe (not sure what brand) and BMW performance power kit and exhaust. Started making this noise after removing and reinstalling the charge pipe. I also replaced the 3rd brake light and installed LED side markers at the same time. The sound seems to be coming from right where the charge pipe is which is also right where the alternator is. It's a very high pitched electrical noise that does change with RPM but not proportionally like a vacuum or boost leak or a belt-related noise. It has a few pitches that it will step up and down through as the RPMs change. It doesn't change with the AC. I don't think it's coming from the rear main, PCV valve, anything like that. Could it be the alternator failing? Battery dying? I tried unplugging the LED side markers thinking maybe they were bad and it was putting a heavy load on the alternator or something like that. The car runs and drives perfectly with no issues except for the noise.

Any help would be much appreciated!

Last edited by kroymander; 01-16-2021 at 11:28 PM..
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      01-17-2021, 09:57 AM   #2
DJ1911
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With the engine off, remove the belt from the alternator pulley and spin the pulley by hand to see if you hear any noise
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      01-17-2021, 11:06 AM   #3
Lester
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Sounds like a bearing or bushing in the Alternator. You might try spraying it with WD40 to see if it stops temporarily. A replacement rebuilt is about $300 so not too bad if it turns out to be that.
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      01-19-2021, 10:47 AM   #4
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Sounds very similar to an issue I posted about, but your noise is a little louder. Does it ever go away?
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      01-19-2021, 10:37 PM   #5
kroymander
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Quote:
Originally Posted by DJ1911 View Post
With the engine off, remove the belt from the alternator pulley and spin the pulley by hand to see if you hear any noise
didn't get a chance to check last weekend but I'll try that this weekend.

Quote:
Originally Posted by Lester View Post
Sounds like a bearing or bushing in the Alternator. You might try spraying it with WD40 to see if it stops temporarily. A replacement rebuilt is about $300 so not too bad if it turns out to be that.
good to know and yeah I like the WD40 idea I'll try that too but wouldn't a bearing or bushing cause a noise that is proportional to engine RPM? this noise really only has a couple of frequencies that it will step up through as the RPMs increase which doesn't really seem like it could be mechanical.

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Originally Posted by jsublime View Post
Sounds very similar to an issue I posted about, but your noise is a little louder. Does it ever go away?
it does not go away, and it doesn't seem to be louder/softer when the car is warm/cold. not affected by steering or brakes. when I put the clutch in at idle the frequency steps up a tad but that's just because the idle raises a tad as well. if I give it a little bit of gas to rev a bit it will do the same.
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      01-31-2021, 06:44 PM   #6
kroymander
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turns out it was the vacuum hose that goes underneath the airbox then around the driver's strut tower and (I think) back to the exhaust flap. came unplugged right at the strut tower and the hose fell down out of sight. finally found it just as I was about ready to take it to a shop. anyway thanks for all the help I hope this thread helps someone else in the future
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      02-02-2021, 11:46 PM   #7
brian360
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Quote:
Originally Posted by kroymander View Post
turns out it was the vacuum hose that goes underneath the airbox then around the driver's strut tower and (I think) back to the exhaust flap. came unplugged right at the strut tower and the hose fell down out of sight. finally found it just as I was about ready to take it to a shop. anyway thanks for all the help I hope this thread helps someone else in the future
I had the same exact noise and it turns out the same small piece of vacuum line had disconnected on mine, too. Thanks for posting this!
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      02-09-2021, 03:44 PM   #8
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Quote:
Originally Posted by brian360 View Post
I had the same exact noise and it turns out the same small piece of vacuum line had disconnected on mine, too. Thanks for posting this!
I'm going to check on this asap, thanks!
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