I had a buzz that developed in my 7700. I traced it to a small Vornado room heater that I have on the same circuit as the 990/7700 units. When I unpluged that small room heater the unit (7700) stopped buzzing.
I would suggest that next time that buzz develops that you start unpluging and or turning off items that are on that same circuit untill you find the culprit as i did.
PapaBear