I had a new Amer. Std. Heritage 15 2.5T heat pump with a TAM 7 air handler installed in the summer. In AC mode there is no abnormal sound. I turned the heat on for the first time about two weeks ago and now when it runs in heat mode I hear a sound that seems to be resonating through the line set. The air handler is in the basement and I can really hear it down there. The heat pump is working fine. The dealer came out and verified this is normal in heat mode. What do you think? Is it normal or not? I have included a link for you to listen to the sound I recorded. The recording was taken right along side the lineset in the baesement. 20 seconds into the audio clip you will hear the outside unit shut off. Thanks for all that listen to the clip and let me know their technical opinion.