Rover

What colour smoke? Black over fueling, white burning coolant, blue burning oil, bad lifters tick, and it will not be constant with engine speed, what do you see and hear?

If it rattles on acceleration and blows black smoke this can be a faulty injector a problem with a knock sensor if used, I would do three things first, run a diagnostic scan and get the fuel rail pressure tested and check the spark plugs for excessive gaps and one or more that are very dirty, black and sooty looking, start here.