If it's a buzzing noise, then it's ignition noise.
Do you have a noise suppression filter on the distributor? Also the noise of the ignition will be picked up by cheaper Audio Power Amps. Do you have a seperate amp?

Try working through this:
Crutchfields troubleshooting process

First you need to determine if it's EFI or RFI.
Electrical or Radio Freq Interference...Or ..is it coming from the wires..or through the air.

If you've got a second battery in reasonable condition put it on the passengerside floor temporarily, and power the radio/cd unit off that. Start up the car, and see if the noise is still there. If it is still there, it can only be RF noise. Caused by a cheap radio or noisy alternator, distributor or spark leads. It's possible that this may not be fixable..especially if it's the headunit.

Upgrading the Engine to a CA18 or SR20 will help get rid of some noises