Originally Posted by Dave Henshall
OR.... its the camshaft caps that have come loose (on the exhaust side more than likely and tyhey wont tighten up again, the heads seem to go soft with the heat and the threads fail if a gorilla has over torqued them they pull their threads out. i had to helicoil 10 of the holes on the exhasut side when i rebuilt my head last. that was ratling quite a bit.
had a think about it today got that in my mind and also the front bearing on the camshaft is a ball bearing type that could make a noise similar to what you described.
anouther one is to wire open the wastegate on the turbo to stop it spooling up to see if that is causing the noise.