Sounds suspiciously like an exhaust flange gasket leak to me. Try to shroud the rear exhaust flange where it meets the head with a wet rag wrapped around it to see if the sound changes or goes away; the rag will absorb the noise from a leaky gasket. If the knock is still there, its probably somewhere inside. I had a stuck lifter that knocked one time, drove me nuts, but when the bolts on the headers got loose, the leak sounded just like that.

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