You need to isolate 100% proof positive the source. I would take off the serp and run for a brief shot to make sure you aren't listening to an air leak and the sound vanishes when the alternator is not spinning.

The power steering pump is right there as well and possibly more likely to make a whirr sound that i'm hearing on the video.

if you had a shorter serp belt some creative winding you could isolate one at a time to determine which accessory is whining.

there are a lot of sounds in that audio but best guess the whistle you are hearing is about 1700Hz at idle and about 3800Hz when you rev.

If you measure the diameter of the crank pulley and the alternator pulley you can compare that; if for example if it's 2 to 1 ratio and the revs are going from 850 idle to 1900 that would make sense if the sound is coming from the alternator but it felt like the rev was much higher than that.

Considering somebody else's recent failure of a fan clutch, check that out ; see if the fan spins freely when engine is off, then I would block the fan with a piece of foam (i've used foam pipe insulation in the past) and block the fan from spinning (if you have a clutch fan of course NOT if electric); what i'm seeing is a sound that changes frequency NOT in proportion to the RPM which would suggest NOT part of the spinning accessories.

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2011 E70 • N55 (me)
2012 E70 • N63 (wife)