hot water knocking

When I turn on the bathroom sink faucet I get a knocking sound. It doesn't happen any where else in the house, just the one sink.

On a faucet that uses washers rather than the more common (today anyways) cartridge, when the washer gets old, sometimes it can vibrate as it loosens up. It may only take replacing the washer. You may also need a new screw to hold it in (some just snap in).