Strange knocking noises from my L D V pilot engine?

Hi there. Just had the engine oil and filter changed on my LDV pilot van. Its a 51 reg 1.9 Peugeot deisel. Drove the van from Blackburn to Bolton and back with a full load and 5 people on board. The van handled fine, with no problems there or back. Its a very steep hill out of Darwen and from Bolton, but the van handled the journey no problem at all. We unloaded the van and as I was making my way home the van started to make what I can only describe as a Knocking noise, in the same time as the rattle the diesel engine normally makes. This noise lasted for approximately half a mile or so then stopped as suddenly as it started. I've since driven the van a number of times without the same noise returning. can anyone suggest what the knocking noise was, I would have thoght a bearing knock would be there all the time, however as you can probably tell I'm not a mechanic so any advice would be greatly welcomed. A bloke at work said something about hydraulic tappets on these engines sometimes take a while to settle after work being done. Cheers Paul.