I've just transplanted a Clarion HU from one car to another.
It never buzzed in the 1st car it was in.

It buzzes while the engine is running only.
I have taken the alternator belt off and run it, it buzzed.
I have added a suppressor box to the ignition wire, buzzed.
I used a relay from the ignition wire to connect it to the battery supply, and it still buzzed.

It currently only runs front speakers off a little Sony amp.
But it doesn't buzz with the engine off.

All power and ground leads are good, battery earth to body is only 0.22 Ohms.

Any ideas, things to try, test or smash, i'm all ears.