My 1994 Astro makes a very loud hissing noise when I put it in gear and under partial load. I'm talking VERY loud hissing. It does not do it when in park even if I bring the revs way up. Only in gear and under load. Has next to no power when it is like this. I cannot find the leak.
Van has propane conversion which seems to be fine. I do not think it is a fuel problem, but a severe vacuum leak.
Any suggestions would be most welcome.