Outside Plumbing Fixture humming / whistling

[swarsa]

Hi,

I'm having a problem with my outside plumbing fixtures humming after I turn the water off (this is an inconsistent problem). Please see this web page with pictures for explanation):

http://www.coolgadgetssoftware.com/PlumbingProb.html

From comparing this picture to other pictures online, it seems to be a 'Backflow Preventer'. I can't figure out what triggers the problem, but turning that knob (in the picture on the above web page) corrects the problem. This problem seems to happen daily.

Thanks,
Steve