A piston installed backwards on the connecting rod, or the entire rod/piston assembly reversed on the crank journal will cause some oddball noises due the the wrist pin offset being turned the wrong direction. The piston is "floating" in the cylinder as the spark plug fires, causing the piston skirt to slap against the cylinder wall.
A main bearing knock generally is loudest as the engine is accelerating, and tends to diminish or disappear as the load comes off the crank just before the transmission is shifted to the next higher gear. Then the knock begins again as the load comes back on. Connecting rod knocks tend to get more noisy as speed increases. Yours sounds most like a main bearing, with a possible piston slap and a rod knock following, in that order.
Jerry
