Is it a side spay or a pull out spayer? A side sprayer faucet sends water to the sprayer, which you're not using it diverts to the spout. My first inkling is that you have too high of water pressure. Anything above 80 is generally above legal. Another thing could be water leaking out of the sprayer and being replaced with air allowing the water to rush to the sprayer handle in the off position and then Hammer once it fully pressurizes and diverts to the spout.