We just purchased a new single-family home and from time-to-time, we will hear a "tapping" or "banging" noise in what seems like the air vent (to the bedroom). Sometimes the noise lasts 5 minutes, other times it lasts for hours. Our house is all electric. Could it be a problem with the furnace or heat pump? Problem with water heater? At this time, I don't want to call a repairman until I have some ideas on what it could possibly be. Please help!