terrible knocking noise

My 2003 td5 was running fine untill whilst driving yesterday there was an unhealthy knocking noise which started when i was towing my caravan back to storage, i stopped as soon as i could and switched it off. it sounded like the fan had come off and was hitting the radiator but it was all intact, i checked all the pulleys and they were fine and the belt still good, there was nothing obvious i was recovered back to my usual garage where i have it serviced. today i called them and they want to take the head off as they think it has dropped a valve! it was still running fine when i switched off could this be right and what sort of damage would it cause?

Andy.

its not the oil pump bolt coming loose is it, was the oil light flickering at all

I dont think the oil light came on but i was in a bit of a flap by then! i hope it is the oil pump.

Andy.