I noticed a hissing noise, that sounded like a vacuum leak, coming from the dash area. It would happen immediately when the car was shut off, and sometimes when you stepped on the gas to accelerate from a steady cruise. I checked all the vacuum capsules that operate the vent flaps, and made sure all the hoses were conncted. Everything seemed to be working properly. It turns out it was the check valve at the manifold, which was doing nothing, and would air to go in the reverse direction under conditions of low or no manifold vacuum, which occur exactly at the times I mentioned. I put on a good used one, and now all is quiet.