My direct-vented propane wall furnace (an Empire DV-35-4SG) has begun making an ear-splitting high-pitched whine as it heats up (the fan is not yet going at this point). The gas comes on, and then it sounds like a bus screeching to a halt. The entire thing lasts 15-20 seconds and has been occurring every heating cycle. My impression is that the metal chamber where the combustion occurs is resonating as it heats up.

I have lived here for 8 years and this is the first year the unit has made any abnormal noise at all. This just started a few days ago, and got progressively louder each day. It is going to cause hearing loss if it keeps up; it is that loud. I would appreciate any suggestions you may have. Thank you.