What's that annoying buzing???

[doug_ngl]

Hi Gang, Decided to run the central vac, so I plugged into the lower
30 amp service. Everything works fine, but unless there is a load on
the service, there is a pronounced buzzing coming from behind the
incoming AC supply switch. It does not do this when I am hooked up to
the upper service...but then again, the battery chargers are running,
so there is a load...any thoughts? Thanks from balmy Gunnison, 5
degrees F above here this AM. Doug Engel, Gunnison, CO 1981
FC35SB "Pokey"