Assuming that you have a zetec (because of the forum we're in) one of the most common areas I've found to leak is the rigid hose that goes from the brake booster to teh intake manifold. I've had several that had a crack about mid way down right before the bend. It's not enough of a leak to be noticed in the brakes, but it will cause a hiss and trip a code. Another area to look is the buddle with the 2 vaccum lines and heater hose that is wraped in a rubber sleave. I've had a couple that the clip for the a/c line had worn through one of the vaccum lines. If you're hearing a hiss, get a piece of hose, put one end to your ear and follow along the vaccum lines with hte other (while the car is running). You'll find it that way.
