These engines are chain driven camshafts, the chains are supposed to last for ever.. .but they dont! if its a rattly/tappy noise comming from the chain side it could well be the problem, good news is, if its just that, the engine is fine and it will just need changing, bad news is changing it is realisticly an engine out jobby.. unless you know some one who enjoys getting frustrated in a very small fiddly space, either way i know its the best parts of £800 to change it with the correct tools etc.. can be done DIY though.
Other problems that cause top end noise on them are the hydraluic lifters, usualy make a tapping sound, its not that unusual for one to block up with a bit of sludge and cause it to fail, or even for one to collapse and fail totally.
Another thing that comes to mind, if its knocking from deep with in... gets worse on acceleration and usualy goes on deccleration.. it could be piston slap, its quite normal if that is the case and it shouldnt cause any problems untill its unbearably loud.
Mabey you could post a video recording of the noise... its a bit easier to tell, even water pump bearings can make nasty noises!
