I'm sorry Kent, I do not have any good suggestions for you. None of my vacuum leaks made noise. The suggestion to pick up a vacuum pump sounds good. You can apply vacuum to particular lines and test for leaks, in this process you may get rid of the noisy part. Here is a guess, and only a quess, under the dash is a black line not connected to anything. This line is a vent line for the system. (The idea is to draw clean cabin air for the vent rather than engine compartment air.) Check to see that this line has no obstructions and is not kinked, I suppose it could hiss if restricted and drawing air into it. Good luck, I will continue thinking about this one.
Rick
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