46 2n 12v with Sherman SU tranny...While pushing/pulling snow the last couple of weeks i've noticed a knocking noise coming from the engine. The noise only rears it's ugly head under load and with the engine above half throttle. The sound seems to be coming from the rear end of the engine, say around cylinder #4 about midway or above on the block.The tractor starts, idles and revs great,no signs of loss of power,can get a small puff of smoke if I jam the throttle full but, only for a split second and clears right up. My bro in-law saw me moving snow and wants to buy the ol girl but I won't sell it to him until I know what is wrong. Not sure where to start looking here any ideas? Thanks Dan