If I set the temperature of the climate control on my 1990 UK TT to anything less than ambient you get a strange loud knocking sound. Sort of 4-5 knocks then 10 seconds silence, repeating.

The knocks are coming from inside the dashboard in the vicinity of the windscreen demister

 

Aircon seems to work OK. I certainly get hot/cold air through the vents as appropriate to the temperature setting

 

Any ideas?

 

Sounds like a dashboard off job. Is this a DIY job and if so are there any instructions on how to do it. How long should it take. Might sound tight but she's now at an age where any major costs mean a trip to the scrapyard. Sob :(

maybe servo motors that open and shut the various vents to when controlled on auto - the auto aircon section of the manual is enormouse - could do a phd on it.

 

Maybe you have some leaves or something stuck in a vent and its fouling the door flaps and stopping the servos - just an idea

 

or a little animal in there that doesnt like the cold:)