Never heard an amp do this before, ideas?

@oldaudiophile led lights on dimmers, a WiFi router, hvac, and refrigerator. A few switching power adapters plugged in throughout the apartment. Florescent lights in the bathroom.

All of these things are suspect to me, most are pretty far from the unit. I've shut down breakers and made sure lights/hvac/wifi/refrigerator are off without effect, though.

​​​​​​I'm in an apartment on the 4th floor in NYC. About 8 floors up is some Verizon equipment. Across the street there is a big service elevator.

part of me is thinking I should find a way to locate the RFI noise somehow, but I'm not sure how.

Do occupants of nearby apartments in the building hear the same with their stereos?

Bring the unit to another location and give it a whirl - I had something similar happen with a Mesa Boogie Dual Rectifier 4x10 "Blue Angel".  Ultimately in my situation there were two problems - one was RFI at my location (which was clear after moving the amp) and the other was a bad capacitor. A quick repair to address the 'singing cap' and we were good to go (especially once I moved to a new apartment and the amp had a friendlier environment).  Unfortunately, I sold the amp to pay for my first wife's engagement ring... woof, did I get it wrong with that one lol :D :D :D

Several years ago I was walking around my house, when all of a sudden two people seemed to have a conversation in my music room. I grabbed a hideout, and went in. Nobody there, then they started talking again. It was coming from a Mission 707S speaker I had sitting on my main speakers. Just as clear as a real person speaking to me face to face. Listening to the conversation, I discovered it was the local police transmitting back and forth from base to cruiser. I looked outside, and there was a marked car sitting in the parking lot of a Pizza Hut across the street. The speaker wasn't hooked up to anything, except for the wire hanging down. I guess it was acting as an antenna. Freaked me out! 

@hobbes101 Wish I had an easy answer for you. I understand the anguish. The reason I asked about electronic equipment in proximity to the amp is because of a similar annoying issue I experienced that turned out to be caused by a DirecTV satellite receiver. I guess those things are really poorly insulated. Even though it was a few feet away from my amp on the equipment rack, it was causing an annoying low level, barely audible hum I could hear through my speakers whenever the volume on the amp was turned up relatively high (music or no music playing). As soon as I got rid of DirecTV, problem solved! My WIFI is on my stereo rack's top shelf, about 3 feet away from my amp on the bottom shelf. No problem!  My 55" LG LED TV is right above my rack. No problem! I've heard of light dimmer switches causing interference. I doubt the rest of the things you mentioned could be a culprit unless the wiring in your apartment building is really ancient. Best advice I can think of is to higher an electrician (a really good/smart one) to come have a look.  Good luck! Just out of curiosity, I'd be interested in knowing what this turns out to be. Even with so called expert advice from a number of accomplished audiophiles, I had a devil of a time finally figuring out my DirecTV satellite receiver problem.