My saab makes a sound like air coming out of a balloon when you brake sometimes… It usally happens at lower speeds and almost always after driving at highway sppeds and then coming off exit and driving a little ways. It is not a squeal or a grind… low grade whitles like air coming out of a ballon.
I assume a vacumn leak… but mechanics can’t find it
It sounds (no pun intended) like your power brake booster has a leak.
Does the firmness of the brake pedal change as you depress it?
I agree with the leaking power brake booster. A good mechanic has ways of testing this. Find a good mechanic.