When I use my air conditioning I can hear this knocking coming from inside that black plastic box below the dash. The AC works fine its just everytime I use it I can hear that noise, what could it be?

 

dunno mine just started doing this too, I think that it's just the blend door moving. I have been thinking my truck has a vac leak for a while which means I might have that problem.

 

The blend doors have electric motors, not vacuum actuated FYI.

 

The AC works great its just its makes a quiet knocking sound.

 

blower motor. does the compressor actually have to be on?

 

mine does it also and the compressor does not have to be on to do it

 

It is your actuator that moves the blender door. Either your door is warped or the calibration for the actuator is off. I have been dealing with a lot of AC issues with my "new" Tahoe. I have already been through this. GL

 

Is it going to hurt anything?