Squeeling sound in speakers 2003 G35

[lemmy]

My girlfriend has a 2003 G35 sedan with the standard stereo (6 CD changer, cassette). Sometimes (usually when the car is first started) there is a very very loud squealing sound coming through the speakers. When the volume is turned down all the way it goes down, but doesn't go away, although it seems to be more in the front speakers when turned down. Even if the radio is powered off there is still some sound but it is greatly reduced. If the engine is turned off, but the sound stops even if the radio is still on. When the car is started the sound will sometimes come back, but not always. After a while it will stop. I have no idea what could cause this. I have searched the forums but not found any other posts concerning this. Thanks for any suggestions.

EDIT: This happens regardless of listening to radio or CD.