[Chris Stewart]

Just to be sure the blend door motor isn't acting up, does the noise change if you slowly change the heater temperature? What if you change heater fan speeds?
I read that with the motor stone cold, if you park so the front/passenger side is at a higher elevation than the rear/drivers side, the cooling system can be "burped" which helps get rid of any air trapped in the cooling system. Take the radiator cap off and let the engine warm while you keep topping off the radiator coolant level. While you're at it, you might install a new radiator cap to be sure you're not hearing the 'ol junk radiator cap gurgle".