A vent in the floor of my guest bathroom has had a soft humming sound coming through it. The A/C will NOT even be running when I hear this humming. It is the only vent making this sound. I thought maybe it was the toilet, but everyone is saying it is the vent. HELP!!

Any sound coming from the air handler? Duct work can transmit noise as well as air?

You might be hearing the transformer in the furnace.