Sounds like a diesel, It does sound like tappets, and I would rule out a blockage if it clears but does it again next start. if it only does it when the engine is cold, I would look at an oil change and go for a good quality synthetic. have you had colder weather than normal recently? If it happens at every start after being stopped for 30mins or more, you may have a problem with oil pressure, and you may need to look at the oil pump, does it come up to pressure quickly?
Good luck, and please let us know when you find the answer in case it happens to others.
