1998 Volvo S70 Tapping noise from under dash whill AC is ru

Air Conditioning problem
1998 Volvo S70 Manual 136000 miles

Hey

My S70 started tapping from under the dash when the passenger side temperature knob is in any position but the highest and lowest setting while the AC is turned on. It sounds like as if I were to take my index finger and just tapped on the dash. It goes away sometimes unexpectedly then it returns. It seems like I can slowly work it to a middle temp setting from either side and it does not cause the tapping. But then again it may return after a while any way. Thanks for your help. Oh and it is not the blower.

Mike