We have a Lennox furnace from 1973. It is an ES2. There is a loud rumbling noise continuosly while the blower is running. It seems to be working fine otherwise. My husband has taken the cover off and cleaned it all out and it still continues. The only way we can make it stop is leave the door to the furnace room open or leave the cover off. The manuel we have for it is basically worthless and we were trying to keep from calling a repairman. We don't really want to put any money into this until we are able to purchase a new furnace. Any help at all would be greatly appreciated.