magoogoo

After racking my brains a bit more Ive remembered the issue. It is most likely your rear A/C unit door actuator clicking . Try backing the rear control head a smiggen off of full cold/hot if it is set to that. If you dont the actuator will continue to try to drive itself until it times out or strips itself. If your rear unit doesnt operate properly then remove the access panel and look for two square plastic white clearish rectangles about 2x3". One is on the top of the unit and the other is on the side.

This is from memory but I believe the part number is YL5H-19E616-AA, the base 19E616 is correct for sure and the base is the same for both actuators.

I hope this was helpful if not please post and let me know. next time i womt snap answer without thinking more about it.