So, this past spring i had a new heat pump/furnace put in. It's been nothing short of amazing. bills are WAY lower thus far compared to last year at this time. hell, keeping the house @ 72* all sumer cost at most 35 / month! (1700sq ft house). anyhow, now that its cold, it makes this rather odd noise. It sounds like a damn dinosaur is roaring in my side yard! My only guess is that when the system goes into defrost mode, it has to switch directions of flow, which means pressures need to equalize before it can happen. is the noise an internal pressure release (obviously can't vent to the atmosphere)? the noise will happen, and then 30 minutes later happens again. but then its gone for a while. this is why i believe it to be defrost as well, because the defrost cycle will run for 30/60/90 min, depending on setup.