knocking noise from under the dash(91 chev truck)

i am having the same problem exactly as you described on my 91 gmc, i have been told something about the temp control or something like that, but it seems noone knows for sure, i cant figure it out.

I messed around with the controls and it seems that it has something to do with the recirculate button. It doesn't knock when the rec button is pushed in. Try this on yours and let me no if it quits.

i seen your message, did this take care of the problem? the REC button, i thought was to just recirculate the air from inside, i dont really care about the noise, i would just like an answer on what may be wrong and if it can be fixed, right now i dont have time to mess with it, right now the frame was rusted enough, that the idler arm(i think that what it is) that bolts to the frame on pass. side, pulled the bolts out, and now i have no steering, i have temporarily fixed it, but not safe, so i need to find out if there is something else wrong to put so much force on these bolts to rip them out.stay in touch on the heater matter, thanx, Dan

Sorry
I haven't had time to mess with it either. I'm going to try and unhook the rec. and see if it gets rid of the clicking sound.