See Three

I have a 71 BB with A/C and I've noticed that when the climate control ***** are on the vent or A/C setting I hear what sounds like a small blast of air coming from somewhere outside every few seconds. Almost sounds like the pressure relief noise that air brakes make. Is there some kind of vacuum relief valve that makes this noise?

MelWff

have you opened the hood with the engine running and AC on to try and determine where the noise is coming from?

See Three

So it was happening in the AC position and the noise was the compressor cycling on and off every few seconds. I took it in for service and they topped it up with refrigerant but 2 days later same thing so system must have a leak.