Have you connected the 7700 to the power conditioner to see if that helps? Here is a long shot. Try leaving the 7700 on and one by opening each of the breakers in your main power panel (except of course the ones feeding the 7700) including the 220 volt breakers. At this point since we have kind of ruled out the 7700 it has to be something upstream. Maybe some load on your power panel is causing a feedback to the panel. If the noise stops, then reclose that breaker and continue the process to make sure there are not two issues. If you find that the noise stops then figure out what is on that breaker and unplug it/them one at a time.