Thumping noise from rear a/c-heater

I have a 2003 explorerer that has a strange thumping noise coming from the rear hvac unit.The best way I can describe it is-loudest when rear a/c or heater is on,but does not go totally away when rear a/c-heater is shut off.noise does not totally go away till key is in off position.Engine does not have to be running for noise to appear.Any thoughts or help would be appreciated. Could it be a blend door noise?