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Whistling noise while pressing gas

I have a quick question about something that probably isn’t anything but when I’m driving and I step on the gas I hear this high pitched whistle. I notice when I get to about 30mph it seems to disappear. It isn’t loud at all but its definitely there. I had my wife tap the gas while it was in park so I could see where its coming from and its definitely coming from the side of the engine where the belt is. The car is fine otherwise other than the battery draining overnight. If anyone has any ideas as to what it is I’d love to know. Thanks.

Car is a 2002 Chevrolet cavalier with 67k miles.

If you don’t know if it’s ever been done, have the serpentine belt replaced.

Then at the same time, the various pulley’s and their bearings that come in contact with belt can be inspected.

Tester

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The battery is draining overnight but your first concern is a whistle? The whistle could be the serpentine belt or a pulley or it could be a vacuum leak.

Either go to a shop or get yourself a volt meter and google “How to find a parasitic draw.”

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… and a slipping belt might explain why the battery is apparently not being adequately charged…

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It could be a bad diode in the alternator. I had a similar problem in a 1978.Oldsmobile I once owned. I ignored the noise for a couple of days and then the warning light came on indicating that the alternator wasn’t charging the battery.
Your alternator may not be charging the battery sufficiently which may be why it appears to be draining overnight.

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Yes!
That is another possibility.