I had a new sink put in about 6 months ago. All was well, until now. When I turn on the hot water it will run for about 30 seconds and then it starts knocking and the waterflow is constricked to a trickle. The cold water works fine. Any suggestions?

for starters remove the aerator and turn it on and see if that helps.
sometimes debris gets caught in there.
post back.

The problem is caused by something that can move around in the path of the hot water flow. When it moves it blocks the flow of water.

Most commonly this is caused by a loose washer or a washer that is not designed for hot water if you have an old 2 hand faucet.

But it can be caused by a bad stop valve or "crud" in the supplyline or faucet.