I have a 20+ year old Trane XL80 Furnace. Within the last few days, I have detected that the inducer fan (upper fan, not the blower fan) is always running. The furnace heats and ignites fine, normal heat cycle when the thermostat calls for heat. The (past) normal behavior is for the inducer fan to come on when it the furnace calls for heat, and then shuts off after the system cools off. Looking for a pointer to what the issue might be, or things to check before I place a service call. Inducer fan runs even when thermostat is set to "off".