I have a 21 yr old Bryant furnace. Recently it started making a horrible noise when the heat first kicks on. It sounds like someone playing the tuba. It only lasts a few seconds, but it's loud enough to wake me up at night & make the dog bark! It happens 75% of the time (mostly at night when the thermostat is turned down to 65). The other 25% of the time (when it's a little warmer), it doesn't happen at all or it's not quite as loud. What could be causing it ?