W202 strange noise fron heater/AC

[elegance4]

Hello list, good day ! Great site, lot's of info. This is my first post since joining a year ago. Was wondering if anyone can help me out with this. The other day I noticed a very faint thud/thump from W202 with 54k when the climate control is on. The sound is so soft that I can only hear it with windows closed and total silence. The sound seems to come from behind the console close to the passenger's foot well. It's very faint and comes at an interval of every 5 seconds. It only appears in heat mode and not in A/C mode. The climate controls works wonderfully except for this noise. It disappears when I switch off the climate control or when I slowly lower the temp setting until the flap moves to cool air. BTW, I have the early pushbutton control panel, not the DIN type.

Not sure if this is related, but I guess I have vacuum leak somewhere because the vacuum pump in the trunk cycles itself 4 to 5 times after starting the car. The leak is very minor as I can lock/unlock the doors even after two weeks of not starting the car.

Question: Where could the noise be coming from. flaps ? solenoid valve ? It is normal ? Can I just be hearing things because the heater flaps are open ? Any long term damaging effect if left as it is ? Thanks in advance !

Willy