If the knock is coming from under the dash it could be one of the air distribution flaps sticking - I had this a few months ago and was able to diagnose which motor using VCDS (VAG-COM). On investigation I found that the motor was OK but the operating lever had jammed.
Maybe the flap closing, try it in E mode. Does it only do it when the recircluating button is on?
If the knock is coming from under the dash it could be one of the air distribution flaps sticking - I had this a few months ago and was able to diagnose which motor using VCDS (VAG-COM). On investigation I found that the motor was OK but the operating lever had jammed.
Problem found with the air recirculation motor,
:icon_thumright:Maybe the flap closing, try it in E mode. Does it only do it when the recircluating button is on?
Dave,
how did you cure this what methods were used to un jam!!!!! as mine doing the same and wont de -mist windscreen help!! Vag says flapp jamed....