[Shotimes] Electrical issues

[Ryan M Dudek]

Vehicle is a 1990 SHO, having some electrical interference type issues.
Seems that the car is getting a nice high pitched whine through the
speakers.  Sometimes it goes away when the radio is off, other times it
keeps on doing its thing regardless of whether there is power to the radio.
There is an aftermarket headunit but no other wiring has been changed to the
best of my knowledge.
Any common causes that you can throw my way would be appreciated.

Ryan Dudek
98 Black - mine
94 Opal Frost ATX - girlfriend's