I don't think it's normal because our 97 'burb makes no such noise. Certainly not to the degree that it sounds abnormal. There's a rather long hiss when you turn off the engine with the AC compressor engaged. It can go on for half a minute. I'm told that's normal.

I have to wonder if it's a vacuum system, though. Our 6.5TD engine vacuum pump serves only the wastegate actuator. Brakes have hydraulic assist. Maybe there's an electric vacuum pump for climate control.

Sixto
87 300D