Our gas furnace has started making a pulsing/humming, very low frequency noise. The noise starts before it's warm enough for the blower to kick on - ie. when the heating call is issued, a flame is present, DI motor is on, but no blower motor. Even after the blower is running the sound is present, but helps to rule out the blower.
The frequency is so low it's on the borderline of being inaudible, but you can definitely feel it. It's becoming an issue because you can feel this pressure through the pillows when trying to sleep at night. It's like a humming noise that pulses in and out over the span of 1-2 seconds.
From some research, it sounds like the most likely culprits could be:
1) flame instability, likely due to gas valve going bad
2) DI motor going bad
3) 24v transformer
Furnace is a ComfortMaker GNE100F14A1. Any thoughts on how to diagnose or additional items to check out? At this point, my only idea is to use a long screwdriver as a mechanic's stethoscope and put it on the DI motor housing and 24v transformer to see if I can pinpoint it that way haha.