This is a long shot, but check the 90 degree vacuum hose that runs from the valve cover (opposite the #4 spark plug wire) to the intake manifold. It is a L shaped hose about 4" long, right on top of the engine, easy to get to. I've had 2 of them crack and that will cause a big vacuum leak and a hissing noise under the hood. Confirm the ends are on tight and check for cracks on the underside. The crack often cannot be seen by just looking at it from the top.
'88 300E 265K
'91 300TE 155K