My furnace is from the 80's and... okay this is going to seem like a stupid question, but where the heck is the filter? We can't find it. It makes me worry that we've been going all this time without one installed. Any idea where I could find a manual and diagrams for this?
Is the furnace in your basement or attic? If in the basement then it has a return air duct attached to the side of it and the filter should slide in a rack there or be inside the fan compartment (not sure if Trane used that idea). If it is that old you should have a tech check the heat exchanger etc and he can show you the filter.