Power supply filter capacitors in the radio are getting old might be it. Does it also do it when the turn signals go on/off or the wipers? Every time there is a change in load on the power in the vehicle there will be some "noise" generated. Electrolytic capacitors lose capacitance as they age. It is what kills many cheap electronic devices as they age. You could try placing a capacitor across the power input to the radio and see if it goes away. https://www.youtube.com/watch?v=NHVvkSbsr_Y
They are polarized so you have to be sure to put it across the power input with + to + and - to -. - is ground in the Fiero.