"Fwaash", apparently, is the sound of the Green Lantern ring when it spouts forth a wash of energy. When the GL ring is used to make constructs, it's silent.
This fwaashing thing is new. It started with Alan Scott, whose power is all flame-y and not construct-y. When he was brought back with the rest of the JSA, artists and writers start soundtracking his use of the ring-energy, usually with a "fwaash" of some sort. Presumably, this was to help distinguish his use of the ring's power from the GL Corps; Alan's power was more like noisy liquid flame, theirs was more like silent solid light.
But now, from what I can tell, when the Corps Lanterns use their rings like fire hoses of energy, they too get the "fwaash" sound. So, something that was originally used to distinguish the Golden Age GL from the Silver/Bronze Age GLs is now used to make them seem more similar.