Here's a quote I found interesting from David (APi engines)..
Assuming the noise doesn't fade after a few minutes from start up it is probably gummed up or weak hydraulic lifters. It is worth trying to clean them up with a simple but effective remedy.
Drain out half a litre of oil from the engine and refill using 1/2 litre of ATF auto transmission fluid. This marketed by a number of companies as ' hydraulic lifter cleaner' and you'll pay a tenner or so for about a cupful.......
My way should be rather cheaper and just as effective.
After 300 miles with the cleaner in drain off all the oil and refill with oil of choice and a fresh filter. Try not to red line the engine during the 300 miles just drive ' normally'