I have read a few posts about this same issue, but have not seen any real answers. On my 1999 Beetle 2.0, I have a hissing sound coming from the full length of the vacuum hose going to the bottom of what I understand to be the PCV heating element which is located on the air intake tube just before the TB. My assumption is that there is an internal leak in that unit allowing it to draw a steady flow of vacuum through it rather than pulling a vacuum against it. Does that make sense? What is the purpose of that vacuum line if that is only a heating element? It almost has to function as a valve of some sort. The hose is good, and when pinched, the hissing will cease.