I noticed this about a week ago, it happens on cold starts, it happens when it is both cold and warm outside, but it doesn't always happen...it goes away when the engine is warm, but it doesn't sound like "normal" suby noise.
Just to be clear on what you are listening for...it's the psst psst psst psst psst psst. It is on the driver side of the engine, and loudest around the AC compressor. My turbo is hitting peak boost, and there is no fluctuation in the vacuum pressure. It does not change when I turn the AC on and off. It does change with engine revs.