HI Everyone,

I just bought this high milage 260E (339k) and I am hearing a rhythmic thumping noise whenever I have the climate control system turned on like a vacuum operated flap keeps opening then closing really quickly.

- Fan works just fine in all modes
- A/C and heat work great
- all vents except the center outside air vents blow air.
- all vent open and close controls are in working order at the vents.

Any ideas about what could be causing this?
Is this as simple as replacing a vacuum actuator?

Thank you in advance for advice!

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1989 260E - Daily driver
1982 300D turbo - gone but fondly remembered.