There are a few different types of transfer swithes. Some hum, some do not, Mine does not (intelletec)
Here is why
On mine the "Resting" state of the switch is connected to shore power.. That is IF NO power is present, Shore or generator, I am connected to the shore power inlet...
If Generator power is present, then when the timer times out, CLUNK and it transfers.
Others have two sets of normally open contactors (Relays, size big) when there is no power the resting state of the unit is NOT CONNECTED.
When shore power is sensed, then ONE set of contactors pulls in, these may have AC coils and those hum.
I do not know why they do it that way.. (I am sure, however, there is a reason, in fact I can think of one but I do not know if it is THE reason).
Another possible source:
I know some companies park the converter near where you cite. (Usually under the fridge) Damon parked the furnace there on mine (Which kind of makes me wonder about their engineers).
Home is where I park it!