I want to start off by saying this is an excellent resource!

We have a late 80s SummerAir forced air electric furance. 125 amp power supply, 4 element furance which was "serviced" one year ago. It was cleaned, and the first sequencer was replaced because it was not coming on consistantly...

Recently, when the furnace first comes on, but not every time, there is a loud, low frequency electrical "humm" from the furance. It sounds like the humm from one of those large electric lights in hockey rinks or outdoor stadiums. Do you have any idea what this is? I don't want to call in the repair man unless this sound can be considered normal...As I said, it does not occur every time, and it lasts about 2 minutes up to 15 minutes.

Thanks for any ideas...