Originally Posted by Alex98
"sound like a coffee can full of marbles".
Definitely Piston Slap not valvetrain slop. There are a few threads about it on RPS. It is known as "Marbling". Some of the 4.0L OHV motors particulary the 1998 have offset wrist pins which cause the piston to wobble at low rpm and load. It is not harmful just annoying. Mileage might have something to do with it. It usually occurs after the truck has sat dormant for a while or after an oil change. If you want to avoid this when you change your oil, change it as fast as you can and put in the drain plug as soon as the oil flow starts dripping. Heavier weight oil or Lucas Oil stabilizer will not help, you want a thinner oil with better flow. To minimize the marbling use a synthetic oil and a higher flow oil filter. The Triton V10 was also notorious for this problem.
My truck is currently not doing this but I have had issues with it in the past. However anytime I adjust the Idle below 600 RPM it marbles like crazy and sounds worse than a 7.3L diesel.