What drives the cam? Belt, chain or gears? I'd take a look at that, it may be moving around.
Do you use high test? If not, I would try some to see if the problem might be pre-ignition at high RPMs.
Check your motor and transmission mounts, in the case of 740 turbos, as the mounts collapse, they get noisier and have more vibrations. Perhaps something is touching.
Consider that some of the transmission spins with the engine, even in neutral, a bad bearing in the transmission can cause quite a racket.
As a last resort, you could try running the engine with the belts off, the accessories can make a lot of noise.
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'96 855R,'64 PV544 driver, '67 P1800 basket case, '95 855, '95 854, the first three are mine, heh, heh, 415,000 miles put on 9 bricks
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