2004 Pontiac Grand Prix Knocking in engine

Ive kept up with all the routine stuff. But when I first start the car its fine but about 5-10 minz into it warming up its a loud knocking sound. When I hit the gas and bring it to 2000 rpms the noise is gone. People I have talked to think it could be the valve on the piston but ive tried all the liquids you put in the oil to try to get them none sticky and put dry gas in but still nothing. I was just wondering if there was anything I can do or try because I dont want to have to take my whole engine apart.