Usually that hiss is caused by failure of the vacuum check valve, located in the engine bay sticking up from the intake manifold. Vacuum is going back to the intake via tubing; the check valve is intended to prevent that.

I've gotten replacements from junkers.

It's an assembly of white nylon plus black rubber tubing. Sticks up from the intake, and ours have a small right-angle metal tube sticking off the check valve.
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Sven: '89 245 NA, 951 ECU, expanded air dam, forward belly pan reaches oem belly pan, open-front airbox, E-fan, 205/65-15 at 50 psi, IPD sways, no a/c-p/s belt, E-Codes, amber front corner reflectors, quad horns, tach, small clock. Wifemobile '89 245