Kitchen faucet noise

Strange thing - My kitchen sink has a two year old Glacier Bay faucet with spray attachment. Recently it has been making a 'thumping' noise at low flow rates - about 3 times per second. I can feel it in the sprayer hose, but not the metal plumbing. As I increase the flow rate, the frequency of the thumps increases, until at max rate it stops completely. It really has me baffled, as there has never been an air hammer 'tube' there in the 14 years I've lived here, so I don't think that would be the cause of the problem. Comments will be appreciated! TIA