Well, my hearing may be a touch better than I give it credit for. Using my Radio Shack analog sound level meter set for "fast" response on the 60 db scale (the most sensitive) with either A or B weighting, this is what I get:

With the meter set up on top of the unit, the meter varies from no change between on and off, to an occasional slight pulsing of the needle between -10 (no sound registering) and -8 db on the scale. The -10 is 10 db below the zero (which represents 60 db), so some of the pulsing may be in the 50 to 52db range, if I'm reading the meter correctly.

Except, of course, for a quick jump from -10 to -4 db when the relay clicks on and off.

In reading the instructions for the meter, it indicates that the lowest (60db) "range of measurement" is 50 to 60 db. I would probably require a more sensitive meter to give an accurate reading of what I faintly hear.

In any event, I wonder if the sound I'm hearing could be due to a variable dirty power thing going on occasionally.

All my previous amps were in some sort of cabinet, so I didn't notice any mechanical noise from them at all, even if they made some.