The 5.9 Magnum in my 1999 2500 pickup tore up a rod bearing right around 85,000 miles on the odometer, and likely caused by me driving too fast in third gear on the freeway with a trailer that was too heavy.
Mine was most noticeable in park and lightly raising the rpm. Isolated the cylinder as oldmanmopar said, by pulling a plug wire off and revving it again. Noise was not heard with plug wire removed from the bad cylinder.
I took the oil pan off hoping to replace the bearing but it was a waste of time. Once I could hear the noise the crank journal was already trashed. Badly scored and undersized.
Do some testing, but know that if it is a rod bearing you should expect to need the crankshaft ground and new bearings. But definitely worth fixing it.
