Your incoming house voltage should always be more than 220 volt. It is typically around 235 or so. You should always have more than 110 volts at your receptacles. It is typically around 117 vac. I would doubt if the voltage is dipping enough to challenge your equipment as the 7700 has a pretty good power supply of its own. I would start looking for a mechanical reason for the hum such as loose feet or a loose cabinet screw. It is possible that the cabinet temperature changes just enough during the year to cause something to be less than tight. Make sure your cabinet is also not the source from loose stuff. A mechanics stethoscope is very helpful in locating mechanical vibrations.