I have a furnance located in my attic to control the upstairs of my home. Only when the heat is on, I hear a loud knocking noise coming from the unit. It does not make this noise while running, but can be heard after it shuts off - it may be immediate or several minutes. Sounds like something is turning and rubbing against some metal. Very loud. It does not make this noise when the AC is on. Any thoughts? While sitting in my attic, I narrowed the noise to come from the unit itself. I orignally thought it was the line coming from the heat pump outside vibrating on wood, but that's not the case. Thanks.