after a cold night,when i go out and start the truck and turn on the heater/ac,there is a loud knocking noise on the inside passenger side from behind the dash.after a while of driving the noise will eventually go away,like arouund 30 mins,but does not happen if i turn off the heater/ac please help its very annoying

You have a “blend door” in the HVAC system that is hanging up and not opening properly.
As a result, the servo motor is repeatedly trying to open it–hence the noise.

Does the rate of knocking change with blower speed?

yes if i turn the blower to high it speed up and down to low it will be almost like someone knocking on the door

Remove the blower motor/fan wheel to see if the plasic fan wheel is coming apart.


Remove your glove box and you should be able to SEE the problem…Perhaps your heater box is full of acorns and the blower is tossing them around. After a while, if moves them out of the way…Perhaps the sleeping rodents themselves are seeking exit points…