Sounds like poor connections in the 12V supply and ground. If you pull the driver's side center console side panel, you'll see - forward by your right ankle - a grounding point where a screw holds some black wires to the trans tunnel. Run a ground from here to your radio/deck and amp grounds. If you are running a deck and amp with some decent power, from the ACC circuit, the factory wiring there is pretty light. My setup only uses the ACC lead to turn on the deck. Power is a constant 12V from a separate (fused) 14ga lead from the battery.

It sounds like you are getting "alternator whine", that's the siren sound as opposed to ignition noise - a buzzy static that rises in pitch with RPM. Step (1) ensure all ignition wires are firmly seated where they should be; (2) sort out the deck/amp power and grounds wiring; (3)last resort is a noise filter on the power lead, available at any car stereo shop.
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Bob (81-244GL B21F, 83-244DL B23F, 94-944 B230FD plus grocery-getter Dodge minivan, MGB, and numerous old motorcycles)