I have heard a similar noise from a 1999 Jetta GL TDI from work. I went to press the brakes, and the noise was emitted, very similar in description to what you heard, andthe brake pedal disapeared into the floor, then came back up. I think it was a vacuum problem.
I have a 2001 VW Jetta Wolfsburg 1.8T. It has almost 100,000kms and
> last week it started making this annoying high pitch noise. It is very
> hard to describe but I can relate it to a hiss from a balloon with a
> very small hole in it where the air comes out realatively slowly. The
> noise comes and goes while driving, can be heard when the engine is
> shut off for about 15 seconds after removing the key; heard without
> any electronics on and sounds like it is coming from behind the
> dashboard. Cannot hear the sound under the hood only in the cabin. >
> If anyone has any ideas, I would gladly entertain your thoughts as
> this is driving me crazy, especially when I drive.
>
> bluezmam