A continuous noise that distinctly speeds up and down with engine speed is due to noise being picked up from the ignition or alternator. It could sound like "ZZZzzzzz" and be more of a fast click at low speeds. Various means can be used to isolate this, like a capacitor at the alternator, or a line filter in the radio power lead.
If it's random or not associated with other accessories or the engine speed, and especially if it does it with the engine off, it's probably something blown in the radio. If it sort of seems to turn on CLICK! off CLIKC! on on some or all of the channels, you've got shorted speaker wires more likely.
Look for bare wires in a door or the wires in the hinges going to the door speakers.
If nothing is found in the wiring, a new 85-89 740 radio ought to be cheap from a junkyard anyway. They're fairly reliable, though the displays do seem to quit long before the electronics die.
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Rob Bareiss, New London CT ::: 86 244DL, 87 244DL, 88 744GLE: 625K total