Believe its related to heater blower. Doesnt happen all the time, When i turn blower off it goes away.
thumping noise coming from under passenger dash. on 2009 Volkswagen Jetta
Probably so debris caught in the fan blades of the blower motor,we see a lot of rodents build nests in the air ducts as well particularly if the car has sat unused for a while. Remove the blower motor and inspect for debris and check the motor bearings and blades.