[ischmitz]

Well, the longer and thinner a wire is between the radio and GND or power the more noise you will pick up. Best to get a thick braided ground strap between the radio and the chasis. And then get a fairly thick (at least 12 gauge) wire to the battery. Then get the supressor and you should be fine.

It could also be that you alternator has an issue. For example I had one blown diode on my original 74 alternator and it caused a very faintly glowing alternator light (only visible in the dark). The car was still charging the battery fine. However, with an oscilloscope I found a very pronounced ripple on my board voltage. And my radio sounded like a turbo-charger. The hum changed with engine speed.

Ingo