Buzzing sound from speakers

Hi

I'm experiencing this buzzing sound when revving through my speakers.

Anyone know what this is? How to resolve it?

I've got a aftermarket head unit in my E34 with a amplifier. Amp is connected straight onto the battery but the head unit takes power from a (+) supply up front but i didn't supply it its own earth. It takes earth from the radio aerial wire.

Buzzing sound from speakers