I just had exactly the same thing happen to me. My 7700 was dead silent then this mysterious hum emulated from the unit. We have a small space heater that we use occasionally downstairs and that was the little culprit. The weird thing is that it's not on the same circuit.

We are having an electric guy come over and put our HT toys on a separate circuit.

This forum is great! I am finding out that just about every "problem" has been discussed at least once and a solution is usually found. It's nice to know that there is a good chance a solution will be found 24/7.