1999 Infiniti I30 hissing sound

Noises problem
1999 Infiniti I30 Automatic 125, 00 miles

I just got my car back from the body shop and, after I turn it off, I hear a hissing sound coming from a valve that's just to the right of the power steering fluid reservoir. The valve is like that on a bicycle tire and is on a pipe that runs from the front to the rear of the engine compartment under the hood. If I put my finger over the end of the valve, I feel pressure and the noise stops. Is this a problem? I never remember hearing the noise before.

The car was having body work done because of an accident and part of the repair was replacing the AC compressor, if that means anything. The damage was all cosmetic, the guy said.