First I would like to thank the opportunity to post on this forum to assess my problem. Have a Heil DC90 furnace. The past couple of days, when it comes on, it has not been makin gthe same consistant noise it has for the past eight years. Instead of a smooth even sound, it sounds like a car engine with the timing off (not as bad as that, but just to give an idea). Heat works fine, and it shuts off and cycles on ok. Live in Michigan, so I checked th outside venting to make sure the inflow of air was okay, and it is. WHen I take the cover off, it seems it may be coming from the Combustion Blower. Just does not have a smooth running sound to it. Curious if there is any maintaince to perform on this, or any ideas on this from any one that is willing to post. Thank you to any that respond.