Actually,it would have knocked a little when u first started it if there was too much clearance.When theres not enough,the piston skirt will expand then rub on the cylinder wall.Once that happens,the piston wants to rock around the bore and it wont be as stable.I didnt see,before,you mentioned that it sounds louder from under the engine,if thats the case,then its definitely an early sign of rod bearing failure.