Differential oil seal issue

Diesel particulate filter (DPF) regeneration is what I suspect you are experiencing. The filter catches soot and every so often it needs to be cleared. The engine management computer is programmed to do this periodically. It makes timing adjustments to the fuel injection system which forces an increase in exhaust temperature to about 600C. This turns all the accumulated soot particles to ash which is expelled thus unclogging the filter. The regeneration typically takes about 10 minutes to run its cycle and usually occurs every 4-500 miles. It will do it more often if you use the car mainly for short journeys where the engine doesn't get up to full operating temperature for long. It is also possible the regeneration cycle won't complete. Sometimes the filter won't unclog and a warning light will appear. If its bad, the engine will go into limp mode to protect itself in which case your mechanic will connect the car to a diagnostics computer and induce a forced regeneration cycle.

The occasional sustained hard thrash along a fast road is always a good idea (my car made me do it, Officer). The 43 guys already know this and they don't even have a DPF >