I posted this some time ago.. Maybe it will help you.. I know that it doesn't sound right, but my hissing came from low vacuum as opposed to too much vacuum. Others were looking at the tank and hose connections behind the glove compartment. After doing the following, I have had no problems for about a year now:
I had a hissing noise which seemed to come from around the glove compartment, and searched all the above described places. I found the hose that runs around to the front of the engine on the passenger side disconnected and thought I had solved my problem. No, it came back after seeming to be "fixed" after about a week. When again checking the check valve, it didn't seem like there was enough vacuum coming through that line - even though it was small. I followed it around to where it comes out of the plastic box (V-6) near the master cylinder. It went into an "L" shaped rubber receptacle which plugged into the "box" on one end with this line and two others on the other end. I finally got the rubber device out of the "box" and found a small flaw in the inside of the "L" and it acted like a flap over the hole going to the line I had been following. I took all three lines out, ran a drill bit (no drill - just the bit) through all the holes and reinserted all three lines completely through the holes and into the larger area at the top of the "L" and reinserted it into the "box". Going back to the check valve, I noticed a greater vacuum. I have not heard the noise since (about 10 months). This problem would probably have resulted in an "unable to duplicate" notation from the dealer, but persistence worked for me. Hope it will help someone else here.
