7.3 diesel death rattle

Hi all diesel experts,

I have a 91 f250 7.3, 130k. On the way home the motor commences to rattle and roll. It goes from running smoothly to rattle rattle pop bang. If it was a gas motor I would be of the opinion that the timing chain had jumped a notch. It will run, but badly. The engine is slapping back and forth but it does run. Where do I start looking?

Thanks in advance, Harry in Iowa