hello before you go into any big cost of pistons and tappets the tappets shold be self adjusting i had a simler prob with mine it had a rattle and sort of ching come from the engine it did not change under reving it got louder on tick over i later found out that it was the locking tab that keeps the oil fillter in it had came loose but it cant drop out due to fly wheel keeps the bolt from dropping in to the engine only found out when changed the oil so try oil change and check this first hope this helps good look