I have a knocking coming from the engine you can.

I have a knocking coming from the engine you can actually feel it knocking when placing your hand on the back (right side) spark plug housing, it has smoked a little bit from that area and the engine shakes like its missing and engine mount but its not when it warms up the shaking subsides pretty sure the cats are bad because of a sulfer smell that hits you when you turn it on I have a pretty big exhaust leak for a while not sure it back pressure played a part in this but the engine runs but I still get a knock/tap out of that back right side spark plug and I noticed its a lil oily around the housing for that back spark plug, all the others were dry what is my problem