Atomic Pi Audio Noise Issue

Good morning folks,
I set up my Atomic Pi yesterday - I've had it for a while but didn't set it up until now (ordered 26th of April). I seem to be getting noise artefacts on HDMI audio out. I've had this with both the original built in EMMC OS, and LXQT from the official download page. It sounds like interference - reacts to screen activity in particular in that it changes tone and pattern. It makes noises like a graphics card with a failing capacitor, but from the sound output rather than from the board.
I've switched HDMI cables, bypassed my HDMI switcher and used the other input on the TV. Nothing else plugged into the same setup does the same thing.
Interestingly, the noise is independent of amplification (doesn't get louder when I turn up the system sound volume).
I can only think that it is either a hardware fault, or excessive noise from the power supply? I'm powering the board with one of these : https://www.ebay.co.uk/itm/5321223337128
I thought perhaps I'd put my power in connection too close to the HDMI jack, but tried bodge shielding it with a piece of aluminium foil and a plastic card (so I didn't short anything) but it made no difference.

Anyone had a similar experience? Or more importantly, anybody know a solution? Feel free to ask for further clarification. By luck, I have a second Atomic Pi landing tomorrow which will be useful for cross comparison / identifying if the issue is on or off board.

Thoughts welcome. Cheers, Phil.

Quick update - interestingly, the issue only occurs when I have my external amplifier connected to the TV. If I unplug it, I can't hear the issue (perhaps a very small amount, but you wouldn't notice unless you were looking for it, and lots of things make a little low level background noise) unlike when it comes through the stereo. Additionally, it even causes the issue when the Atomic Pi is powered on, but you are using another HDMI device (so no video or sound passing through). Only way I could think of it doing this would be if something funny was happening with the ground?

Sounds like a ground loop. Does it go away when you disconnect the antenna/cable from the TV? Then the cable is the problem. There are various products to block such ground loops, e.g.
https://www.amazon.com/dp/B074KD8X6S/ (no i haven't tried this and i blocked my ground loops a different way by having everything grounded and powered by a single power strip/surge-protector with an antenna through.)

Your best bet for noise-free audio on any computer is to use an external USB DAC with balanced outputs to a balanced input preamp or poweramp. Just use the TV for the picture ... HDMI audio is terrible in so many different ways including such ground loop issues...