Mechanical Problem Climatronic

My Climatronic in my Golf IV TDI (215,000 km) is not functioning properly.
Underneath the passenger side, I hear a whistling and a buzzing sound, as if an air duct is not properly sealed. Also, I don't think the front air intakes (Mannanströmer) are delivering full power. When I turn on the recirculation switch, nothing happens.
I can remember that the noise level of the recirculation control was quite different - how about yours?
If I then adjust the value number 13 in the Climatronic menu (which should be the fresh air flap), I sometimes manage to get it to blow correctly again from the air vents. This sounds like the system is suddenly breathing in properly and then completely comes to life.

When testing the strut, you can hear a 'clunk' (passenger footwell), as if something is jumping over, such as a strut or similar.

There are no errors displayed, either when reading the data with the Climatronic itself, or with an external device.
I've had experience with them before, but the mechanic gave such an incompetent impression that I definitely won't be going back there.

Can I be sure that it is the air vent?

If so, can one get there and where is it located?
There have even been people who have had to completely replace their entire dashboard in their VW for just 1000 euros.

Best regards, Malte

So, the air vent cover is located behind the glove compartment. If you start the engine and then press the 'air recirculation' button, you should be able to hear a distinct engine hum and possibly a slight clicking sound.

Have you ever replaced the interior air filter? Not that he is full...

I just had the interior air filter replaced. I also had the air conditioning/ventilation system disinfected because it was making a strange noise in the car. Maybe it's also related to the recirculation issue.
I still need to test the recirculation button.

Greetings
Painted