Almost certainly a bad 'ground' connection from the radio - or from wherever the radio ground is connected and chassis. A resistance to ground develops that is not constant, and the radio amplifiers get positive feedback (the screeching). The simplest solution is to connect a new groundwire from the case of the radio to a known good ground in the car. However, if that bad connection is elsewhere, eg. from the dash framework to the body, other symptoms could show up later as well - but not thru the radio.

Steve

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'91 MB 190E 2.3
'08 RAV4 Ltd 3.5
'83 Lazy Daze m'home 5.7