I finally saw a squirrel's nest in a compressor that looked like a tiny apartment
So I was on a routine service call for a noisy outdoor unit, and the homeowner joked it sounded like a tiny rave. When I popped the cover, this squirrel had made a full home with twigs, leaves, and even a shredded candy wrapper. It had separate areas, like a little living room and a bedroom, which was wild. Anyway, I carefully moved the squirrel family out before checking the wiring, and turns out the noise was just a loose panel rubbing.