Hi - I have an old furnace. It's a natural gas forced air model about 50 years old.
The uint is simplicity itself. There is a gas line that comes into the electric valve and goes to a pilot light and the burners. The thermostat controls the valve and when the unit gets hot there is a thermal switch which turns on the blower.
A few years back it started making a "moan" as it heats up and it's been getting worse as time goes on. This has nothing to do with the blower of the gas valve (the only moving parts in it). When the furnace cycles on, the burners ignite - after several seconds it makes a metallic vibrating sound (reminds me of someone playing a saw with a bow but much lower pitch - about middle C) it's quite loud and it goes on for some time as it warms up. Once it's warm enough for the blower to come on (maybe a minute?) the blower come on and the moaning stops.
I'm assuming the sound is the metal expanding and it stops when the blower come on because the unit achieves some kind of thermal stability (it's not getting warmer with the blower on).
Yes I should replace it - but why is moaning? And can I stop it short of replacing the furnace?