I am in the process of buying a new home. However, I just notice that there is low frequency noise from the hot water heater. I can feel the vibration when I place my hand on the wall in the master bathroom. The water heater is located next to the master bathroom. The builder told me it is the motor attached to the water heater causing the vibration. I want to know why that water heater needs a motor as many water heater doesn't have one.
It sounds like you have a hot water recirculating system in this house. That is where a motor driven pump circulates the hot water through the hot water pipes all the time, so when you turn on the hot tap, you have hot water right now...no waiting. Best idea is to look at the hot water tank and see if there is a motor & Pump conected to it. The builder MAY be talking trash, but it sounds like a circulating system to me.
Hope that is SOME help to ya.