It's part of the regeneration process for the diesel particle filter, the engine management computer is trying to clear the filter of soot.

It sounds like the car needs a run on open roads - try taking the car on an open dual carriageway or motorway and keeping the speed above 40mph as best you can. You don't need to rev the engine hard, just drive at a decent speed for 20 minutes or so. This should help kick-start the regeneration process, and enable it to finish.