Mercedes: SL320 noise, intake leak, waste gate

Question
Hi, I just started to hear a "high-pressure" sounding noise on my 96 SL320. I stopped, looked under the hood, and nothing leaking. Same with the ground(dry). Started moving again and heard it again. It's a high-pressure sounding noise that comes and goes. It stops immediately.... instead of tapering off. The heat gauge doesn't really raise any and AC continues to work. A friend suggested maybe a sticking thermostat? Almost sounds like a pressure release of some kind.

Answer
Rick,
There is a possibility that you have a vacuum or intake leak. Or if the vehicle has a turbo, it may be leaking. If it is in fact a turbo model, the waste gate may be sticking.
Raise hood and start the car "standard precautions observed". Manually rev or have someone inside the vehicle slow rev the engine an listen for the sound.
Good Luck
Rob