I get a hard knocking from behind the glove box when using my heat. not sure what this is. any help? it will sometimes stop while driving but it is related to using my heater.

 

Most likely your blend air door if your hearing a "thump". It moves to change where your heat is coming out, defrost, floor. If the door or actuator wears out it wont stay in position allowing it to move. I'll bet thats what your hearing.

 

your heater fan motor is probably shot, it randomly comes and goes (screeching and banging like a hunk of metal in a drier lol)
when it makes the noise try kicking/hitting the underside of the dash on the pass. side

 

thanks I'll look into both.

 

blower motor