Indefinable "CLUNK' sound (This is a TOUGH one - Experts only) on 1999 Dodge Ram Van 1500

The van is making a single, dull, heavy, ‘thud’ or ‘clunk’ like sound. There does not seem to be any pattern or repeating causitive condition associated with the noise. The sound may occur when the engine is idling or when it is in gear (sometimes when shifting gear, sometimes when accelerating or decelerating, but more commonly when at a constant speed). Sound usually does not occur when braking or cornering.
When the ‘thud’ sounds when the van is in motion there does not appear to be any change in speed/acceleration. Engine (5.2L) is smooth/strong with 93K.

What is this noise and is it a serious issue? Any suggestions for troubleshooting this problem would be much appreciated.

Thank you for your expertise in this matter.

Its hard to guess at the source of this noise with out hearing it first hand. If the noise is experienced when you put the transmission from park to reverse or drive or nuetral to drive or reverse I would look for a weak engine mount. The engine mounts allow the engine to flex as power is applied so that harsh vibration is not passed through the body. If mounts go bad things flex too much allowing the mount bracket to hit its cradled or the exhaust to contact the chassis or body. If the noise just randomly occurs at idle that would be more puzzling, safer get a trained ear listen for the noise.