Whenever my heat is on, no matter what speed the fan is on, im getting this really loud almost grinding noise and im not sure what its coming from because i dont think it does it when cold air is blowing? you guys have any ideas about this

 

This sat. im going to pick up a b5 a4 and it has the same problem but it only does it when its on the AUTO fan setting not the ECON setting. But anyway he had it checked out at his mechanic and he said it was the AUX fan and quoted him 180something to have it replaced. when i pick the car up im going to check it out and see what i can do.

 

Aux fan. Keep econ on, you're fine. Look for anything that might be stressing the fan. What mods do you have?