Its the Tappets (Rocker Arms) on your engine. There used for opening and closing the valves. Valve clearances periodically require adjustment as the contact surfaces of both tappet and valve stem wear. Over time, tappets fail and will break. The problem with this is it no longer closes and opens the valves properly. This causes the affected cylinder to constantly misfire, with the car being noticeably slower than it was previously, the idle speed being lower, copious amounts of white smoke rather than the normal low amount of slightly bluish smoke for petrol engines, and black smoke for diesel engines, and the car sometimes shuddering.
On a V8 engine, the failure of one tappet would not be too noticeable unless you were aiming for high acceleration.
On a V6 engine, the failure of one tappet would be noticeable, as the engine would not be as responsive.
On a 4-cylinder engine the failure will be extremely noticeable, with reduced idle speeds, and increased shuddering as the vehicles' engine attempts to keep running with a quarter of its engine power missing.
On many diesel engines the failure of a tappet would cause the affected valve to contact the top of the [piston], causing major damage to both components, and possible damage to the engine block and cylinder head.
Take it to a garage and they can sort it for you.
Steven Graham, May 2006
