faucet flatulence

Refers to the noisy whooshy spluttering in your pipes when turning on a spigot, often heard when doing so for the first time after having the water turned back on, and so there is lots of accumulated/trapped air in the plumbing that has to be "exhausted".

I just about had a heart attack in first grade when the tap on the sink in the boy's room started making loud popping and banging sounds --- and erratically spewing frothy water --- while I was washing my hands. Well, after all, what with my being only eight years old at the time, and with the washroom's echoey concrete walls and floor amplifying and distorting every little sound, it was only logical that I would have been terrified at this unexpected occurrence --- so much so, in fact, that I frantically called the Principal down to show him. He was totally understanding and sympathetic, gently explaining to me that there was nothing to fear; it was just faucet flatulence.