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Was the engine running before the noise turned up or is this a "dead" engine?
Does anyone know when the engine last ran (if it isn't now) and why it hasn't been running since?
I assume you mean by manually turning that you are turning it over by either starter or wrench on the pulley gland nut. Does the engine turn completely round and round or does it stop dead in conjunction with the sound?
Does removing the plugs make the noise more or less pronounced?
Is the noise close to either top dead center or bottom dead center?
If the noise is not near the ends of crank throw (TDC and BDC) and occurs on either side, then I'd suspect a severely wasted rod bearing on the #2 throw. If you can make the noise by sharply wiggling the crank back and forth (rotate forward and then quickly backward) I'd be more confident. If the noise is near the ends of crank throw it may be a very nasty wrist pin or mangled rod.
The timing gear is the easiest thing to check so removal of the radiator, crank pulley and timing cover should be done before getting too excited. It can make similar noises. Removing the fuel pump and trying again is another nice idea that can help to isolate the timing gear (it exerts a pretty good side load on the cam).
Honestly, if the inside of the engine is pretty gunky and it doesn't run, a noise like that says "tear down" to me. At the very least you may find a "floater" in the oil pan that is causing the trouble.
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Mike!
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