That's un-true unless you are running some seriously wide clearances. Less expensive forged pistons and pistons from yester-year used to have wide tolerances due to the higher expansion rate of the forging compared to a cast piston. For instance; a cast stock piston only needs about .001" clearance between the piston skirt and the cylinder wall where a forged piston might need .005". That's 5 times the clearance but even so, it shouldn't make any "knocking" sounds. On some really quiet engines running older style forged pistons, someone with a keen ear may hear a bit more noise when the engine is cold and nothing has heated-up and expanded yet, but still... it isn't really noticeable.