My gas furnace was recently serviced. I now notice a high pitched hum that is audible when the furnace and fan are operating. Shut the furnace down and the hum disappears. The hum is periodic and not continuous. It is audible throughout the house and it is impossible to isolate the source. I have talked to the service manager. He thinks it is "harmonics" stemming from the rush of air through grates, etc. Not true in my case. Hope you can help. Bill

Where does the noise seem most obvious - near the furnace, or somewhere else in the home? I have seen several cases of "harmonics" produced at the return grilles.