Different type of Vacumn breaker question

This is the only one that causes this. I have vacuum breakers on all my outside hose bibs.
Normally I turn the valves to shut off the water in the hose. But one bib, I usually leave the water on as the hose has a good nozzle.
When the pressure is on this hose, and someone uses water elsewhere in the house, I hear a deep growl.
I'm surmising that as the water is used, the pressure drops and the higher pressure in the hose is activating the breaker.
But why just this one growling. It's loud enough to hear in many parts of the house.
Loosening the cap on the top of the breaker seems to help. How should that cap be adjusted?
----------------------------------------------------- Ya might be a Computer Redneck if Yer mouse keeps knocking over yer spitcan. -----------------------------------------------------
W9OL, Bill H. in Chicagoland
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