Problem with heater/blower

[Ally]

My mk2 172 has something weird going on with the ventilation system. I leave the climate control on auto at about 25degC all the time. After starting the car up initially, the fan speed is very slow for about 30sec, then it blows normally (I assume this is normal).

However, after perhaps 3-4 minutes it sounds like all the air is being diverted to the windscreen vents (it gets very noisy). The display on the climate control doesnt change (same fan speed, same vents in use), and no extra air comes out of the vents onto the windscreen.

It sounds like to me that the air is escaping from the system somewhere behind the dashboard. Does anyone have any other ideas, or heard of this before?

Ta,
Ally

 

[Pete]

Thats normal, its not escaping.

(sh*t I sound like a renault service department saying its normal! lol)

 

[Ally]

Christ it aint half noisy when it happens! When it does it, it only seems to do it for maybe a minute or so, then returns to normal. I ask because its only just started doing it during the past month, and ive had the car for the best part of 8 months now with no problems so far (touchs lots of wood for luck!).