Start unplugging components and plug back in one at a time. Coax for cable, hdmi cables, and Ethernet and like mentioned power cords next to speaker cables all can send signal to a power amp to amplify it. It can be irritating until you can identify the source. “
SOMETIMES it’s this easy. But definitely start there. Ground loops are common.
Sometimes it’s a grounding issue with the house breaker panel. We lost the connection to ours a couple decades ago (in 1972 they used a braided steel connection between ground/neutral on the breaker panel and a copper water pipe) which of course, due to electrolytic corrosion failed and I just last month watched our electrician at our new house in TN connect a braided wire to a copper-plated steel rod with a CLAMP. A STEEL clamp. Iron rust doesn’t conduct electricity - although copper oxides DO.
When I put in my ham radio tower I used thermite gadgets (Erico Cadweld GT1161V One Shot Ceramic disposable body https://a.co/d/jceML9A ) to weld the solid copper wire to the grounding rods and will do this with our new house in Tennessee shortly - right after final inspection - the steel clamps are industry standard (CODE) and are crap (can I say this here?).
Something like THIS is a good investment for starters to make sure your wiring’s right (like HOT black wire to little prong): Klein Tools RT210 Outlet Tester, Receptacle Tester for GFCI / Standard North American AC Electrical Outlets, Detects Common Wiring Problems https://a.co/d/3NI34IS
Good luck, keep us posted.
BTW, anyone looking for the Cadweld gadgets, they’re available elsewhere much cheaper. When I bought ‘em they were about ten bucks (yeah, in THIS century, about 12 years ago).