The noise is somewhat like a rhythmic knocking noise but it is not a metallic, solid sounding noise.
It happens at low engine speed and seems to increase in frequency then disappear with higher engine speed. It is most easy to hear from inside the cabin. It can be heard moving or stopped.
The resonator is the dual crossover resonator which I replaced with a straight piece of tubing welded in place.
I have heard most common engine noises, rod, main bearing, piston etc. and this does not sound like any of those I have ever heard.

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Darrell

1984 300SD "Ole Girl"
207K