Buzzing through speakers. Ground loop?

Hi all, I’m getting a high pitched buzzing through my speakers. It’s higher than 50Hz, but could be a harmonic of 50Hz. My old Yamaha receiver was hard wired with a 2-core cable (not earthed) and it didn’t buzz, but when I tried connecting it to a power amp (3-pin) it buzzed. Now I’ve got a Denon X3700 with a 3-pin plug, and it buzzes.

The setup is simple: C1 projector connected by a long HDMI cable to the X3700, and a Fire stick connected to an input on the X3700. I want to use my power amp for the fronts so I can expand to a 5.4.2 setup, but I’d like to fix the buzzing first.

The C1 is on a different circuit to the rest of the equipment because it’s mounted to the ceiling. Due to the fact that it only seems to buzz with earthed amps, I’m thinking ground loop is the most likely culprit. Do you concur that a ground loop is probably the cause? If so I’ll have to have another GPO put in the ceiling.

I guess I can test it by connecting a long extension and plugging the projector into the same circuit as the rest of the equipment. I’ll report back.