About 3 weeks ago we started to hear a very loud and annoying ringing sound coming from our neighbor's house. My husband determined that it was coming from the heat pump when the air conditioning is running. We contacted them, but we have not received a response. It would be helpful to know what might be causing the problem before we try contacting them again. We have lived here 18 years and this sound has never occurred before, and it wakes us up during the night. Any information would be greatly appreciated.