After about a year a so, all of my units start making noise(hissing) while the unit is on. Does not matter how well I clean the receptacle. You are supposed to turn the unit upside down several times but the metal wires that run the length of the chassis on the inside get worn down. I have bought 7 over the last 5 years. As good as they work, I am starting to get tired having to return them all the time. For that kind of money, they should be problem-free longer.