First off , has the furnace been pressure tested in the last 4 years? The heat exchangers need pressure tested every 4 years and air flapper media changed. The noise you are describing is probably the air diaphram tube rubbing the furnace cabinent. It is behind the rectangular box with the 6 bolts. It takes a service tech to repair this as the box has to be partially removed and the furnace cabinent trimmed to give it clearance. Most techs do not know about this, so show him what I have written. I have had to do this on several furnaces. Only call a Lennox dealer as they are about the only ones who have a pressure testing kit. I would pressure test the heat exchanger first and if it passes check the air diaphram tube. To find out if it is rubbing, the tech can move the housing back and forth to see if it is rubbing after he takes the cover off with the 6 bolts. If he can make it rub, you have found your problem. Hope this sheds some light.