When the unit shuts down it makes a very loud humming noise. It sounds like it is coming from outside and possibly the noise travels through the pipes. The noise last for about 30 seconds or so and then shuts off. We have not had any problems with it heating the house, but worried something is happening to the unit. The noise is very loud. For some reason the noise goes away after the EM heat has been running for a day or so. When the temp goes back up and the heat pump runs for a few days we will start to notice the sound again. Please help?