Driving home from work today (34 degrees out) my heater started making this clunking noise, I turned the heater off and after a few seconds the clunking stopped. About 30 minutes later it started back up again, and after about 30 seconds it stopped. Any ideas as to what I can do about it would helpful. Winter is starting and Colorado is not a place to be with a bum heater.

Mike
Frederick, CO

 

Either an air bubble in the coolant or blend door actuator

 

What does "heater" mean? The fan, the heat exchanger, the heater box. etc
What does "turned off" mean? Switched fan off, switched to AC, switched to recirc, switched to upper vents, switched to defrost. etc
Is there hot air coming out of the selected duct?