I replaced the vacuum booster on my wife's 1996 960 about 1 yr ago. Lately, the dash vents have been closing when the engine is under load. Air flow is high one moment, and then nil as I climb a hill. Let off on the gas, and air flow returns. It's a bit like the wipers on my 1949 Chevy truck.

There is now a moaning sound when accelerating as well, coming from the area of the vacuum booster.

Any ideas on the root cause and a cure?