997.1 2CS windscreen vent issue

[aj-83]

Hi all - I just bought a 997.1 2CS (my first Porsche!) which drives like a charm but I'm experiencing an issue with the interior windscreen vents. At random moments in time they start blowing / releasing hot air in the cabin while producing a fairly loud squeaky noise. I haven't really established a pattern but it seems to happen when slowing down after some (30 minutes or so) highway driving. The AC itself is working fine on auto and manual. Seems as if the engine pushes hot air through some broken seal or so? Any pointers would be highly appreciated; I've only picked up the car a few days ago so I'm keen to get back to the seller with a clear story to see what we can figure out.