I have a citroen relay 2.2 hdi 2007, and when cold i start it as normal and it starts immediatly every time, however the van has a missfire at idle only when cold, it clears as i increase the revs to say above 1500rpm, once warm its totally gone even at idle, the van pulls well, also regardless whether its hot or cold on idle i have a Diesel knock which to me seems louder that usual, has anyone got any ideas what may be causing this?, please note i have had a code reader cheack for fault codes and there are none present