Hey guys I was at a freinds house and they started there forced air natural gas furnace and it made a loud humming noise I know it wasn't the fan it sounded like it's comming from the heat exchanger the burner flames looked good.
I have never heard this before I wounder if this is a sign of a bad heat exchanger.
I didn't have any tools and I mainly work on commercial equipment so I don't have a huge amount of experience on furnaces and if it could be dangeres I'd like to let them know.
I've heard a harmonic in cold HXers before.
did it go away after things warmed up?
thanks for the reply