'99 Chevy Tahoe Electrical Issue

Hi all, I drive a 1999 Chevy Tahoe which I believe has an electrical problem. A faint but distinct high-pitched whistle is audible whenever the engine's rpms are high, almost like a turbo-charger. Additionally,
the interior dash lights flicker slightly at the same time. I have an iPod radio transmitter which picks up this whistle and amplifies it (much to my dismay) when I listen to music, so I'm sure the problem is somewhere in the electrical system.
What are the possible causes? I have replaced the alternator, so that shouldn't be it. This is driving me crazy!
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check carefully and make sure all the Grounds are good(connected snugly,no rust or corrosion)Check all grounds(ie negative terminal to body,engine to body,engine to frame,radio ground,hmmmm,you say your lites flicker?maybe its a bad ground for the lighting system.I bet you someone with a little knowhow and a multimeter might be able to track it down...in time.
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