Buzzin, just can’t nail it

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.

check your ground, try grounding it directly, if you use factory grounds they tend to go through the engine bay and can pick up interference from the ignition system

I agree with Darren, try using a hard ground to the chassis of the car. The engine compartment, even the dashboard of a newer car is so electrically noisy its not even funny.
My last car, a '92 Buick Regal picked up interference from the fuel injection system until I grounded the receiver to the frame.

The head unit is grounded to a bolt that holds the steering column in at the body. The amp is grounded to a bolt in the boot that goes to a leaf spring.

Try running a ground wire for the HU all the way to the negative battery terminal.  Also, make sure you do not have any cuts or nicks in your ignition system (spark plug and coil) wires, no cracked spark plugs, no cracks in your distributor cap, and the ignition system wiring is "suppressor" type.

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The steering column is not a great place to ground.  I wouldn't use a leaf spring bolt either.  I'd make my own ground for each- scrape away paint and use a star washer.  You can try linking the two grounds together.  If you power the speakers off the deck do you still have noise?

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My ex once told me I have a perfect face for radio.

I have just started working at an electronics manufacturer as a tech. I am not a 100% pro, but you may want to consider the following:

Are your RCA cables shielded? You may be experiencing ground loop? I don't fully understand ground loop yet, but maybe if you shield your signal wires it will go away?

Just a thought.

I have been fiddling around and made some progress only to find two seperate problems.
The ignition hum has been cured via a 40A 3 wire noise suppressor across the battery that feeds the distrobution block. And a little help from a 2200uFd and 0.47uFd polorised capacitors from hotwire to earth.
The other problem I will start a seperate thread for and this topic is free to be removed at any time.