My pld merc sounded awful and was getting a bit worried until i was told it was just 'diesel knock' and can be cured with a small adjustment. In the mean time i thought i would try some veggy oil, i added 6 litres 100% rapeseed on top of about 12 litres of diesel. Within 2 miles the engine became so quite on tickover i thought it was going to die. Done another 5 or so miles and all is well, i've got a new fuel filter so perhaps this should go in asap just incase?