aripurdy

Just bought a 99' Outback Limited from police auction. It runs well, all fluids are normal, head gasket has been replaced same with the timing belt. But I've noticed that occasionally the engine makes a knocking noise and I have no idea what that means. Everything seems to run right but everyone in awhile it knocks and I'm not sure if I need to be concerned or not. I plan on taking it in, but I was hoping to get a better sense of what the issue could be. It seems to knock only when I let off the gas, doesn't do it any other time.