Climate control hissing

[mach0415]

Lately, the Climate control area makes a hissing noise when the temp wheel is on anything other than max cool, unless on auto - once it reaches selected temp, it stops hissing then, too. My guess is a change over valve or pod. BUT, I also noticed that when vacuum is leaking, the leak slows down and stops under part load/part throttle! Is there a vacuum pump function that slows/stops vacuum under load??? It almost reminds me of how gassers lose vacuum under load.