You wrote that no wires were frayed or touching the chassis- did you reference any of the speaker wires to the building's electrical ground or neutral?

You also wrote that you disconnected all of the sources- is it safe to assume that you still had this buzz after doing that? If you did, which source inputs did you use- only analog inputs, or did you check the digital inputs, too? If it makes this noise with all inputs, did you move the AVR? One way to find out if a noise is RFI or EMI is to rotate the chassis 90 degrees to the left or right- if a noise source is strong enough that it gets through the shielding, you need to move the AVR or the source.

If you have headphones, disconnect all of the speaker wires and check for noise.

WRT your comment about ground loops- if no sources are connected and you still have noise, you have a bad AVR IF there's no noise source nearby. If you have a wire toner, use the wand to check for EMI in the area of the AVR- press the button and wave it around the chassis, then move it away when you hear this noise.

If you're using a UPS for the network, a DVR or NVR or some other device, make sure it's not near the AVR- I had a problem that seems similar to yours- it only appeared after I connected an additional set of input cables that were coming from a detached shed and the noise wasn't on those, it was coming from the UPS. If I connected those cables to ANY input, I heard noise but if the UPS was turned off, it stopped.