250-500cc Knock in engine... maybe? Help please!

I have 04 wr250 it runs really well. Today I was playing with the idle, the idle tends to hang, think it just needs a turn on the air/gas screw. Anyways I was riding it around a circle dirt driveway revving it up pretty high and popping clutch around the turns, but when I pulled the clutch back in and the rpms went down I would hear a single knock, like two pieces of wood banging together. So I then was just revving in neutral and every so often I’d hear it again, just one single knock. I’m pretty sure it’s coming from the engine, but otherwise it sounds good and rides fine. Any ideas on what it could be?