Had oil changed recently and the lifters replaced. I noticed an occasional knocking sound.
A knocking sound is not a good sound to have. It could happen in a number of places. It could either be in the valve train or could be a piston rod or crank bearing worn out. Either one is not good. I would recommend taking it to a mechanic and have them diagnose the noise. It could also be a piston slap, either one is going to take a complete rebuild of the engine to fix.