Temporary knocking from a diesel?

[bobthebuilder]

Just turned into a junction and there's a terrible low frequency knocking from the engine. Turn off, no obvious lose metal work or anything in the engine bay so fire up again and the knocking has gone!

Drive off for 100 yards and then at low revs is starts again, but only at idling speed. Turn off, start again and it's gone again. I've now driven three miles whilst looking for a garage and no repeat of the knocking.

So rather than go to a garage and ask about a fault that's not currently there, anyone got any ideas? I'm currently on holiday in Weston Super Mare to make things even worse.