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Old 07-26-2021, 01:31 PM
oldskewel oldskewel is offline
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Bearings can make sounds like that, that come and go very abruptly. The instant on / off is an important clue here. Foot off the gas would let the engine spin down, changing the drive belt tension, which might trigger an alignment shift in a worn, dry bearing, that might turn the sound on instantly.

Is there any way you can recreate the sound while it is parked? If not, that's an important clue too. And it will be tougher to diagnose.

Sounds in general can be really tough to trace. Has the problem progressed to the point where it is exactly predictable when it will occur, and if so, what is the easiest way to get it into that state? For example, easiest would be if you need to drive for 30 minutes, then park it, pop the hood, and rev and let off the engine.

Belt off is a great way to quickly rule a bunch of stuff in or out if you are able to conduct that test. Belt runs the water pump, so you won't be able to drive it 30 minutes, for example. PS too, so don't count on making sharp turns. With no alternator spinning, the battery will be lucky to last 30 minutes. Everything will do fine in park though, for a few minutes.

Have you tried shifting to neutral while it's happening? Shifting gears manually?

Is it definitely coming from the front / engine / front differential area?

And on the "whistling" - I once had a bearing failure that sounded like that - I would have sworn there was gas or liquid involved, but it was a worn bearing (fan bearing on an old Lexus).

Another time, there was a high pitched whistling that would only occur when going over 80 mph and maintaining speed (neither accelerating nor coasting), and it would go away instantly when I'd slightly roll down a window, trying to hear better. I was convinced it was some aerodynamic / air pressure effect. That turned out to be the automatic transmission. . Gave up trying to figure it out, and then we noticed it went away after the AT was replaced. LOL, so now you know why I'm so cautious about believing things.
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2001 X5 3.0i, 203k miles, AT, owned since 2014
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