Rattling noise with the SOHC V6 is typically engine timing chain related. Remove the lower oil pan and check for broken pieces of tan plastic. If you find them, they are from broken timing chain cassettes. Does it rattle (make noise) at cold start-up and around 2700 RPM while accelerating?
SOHC rattling is the beginning of the end of your engine unless the timing chains are addressed. There are probably a thousand threads on this subject.
Edit:
Something else that can make noise is a broken weld on one of these exhaust shields, though I wouldn't classify that noise as a rattle, more of a vibrating noise. A loose brake backing plate can also make noise. Rule these things out first if you think this might be the source of your noise. Timing chain rattle sounds more like a diesel engine rattle.
If you don't see any broken plastic in the oil pan, the rattling timing chains might still be the cause of the noise because your chain tensioners are weak and/or the chains have stretched beyond the tensioners ability to take up the slack. If it;s the rear chain (passenger side of engine) that's making noise there is a possible fairly easy fix you can do by installing a manually adjustable chain tensioner. Search for "Polaris". In any event, timing chain rattle is serious and if let go the chain can jump-time and bend your valves, or even break. Replacing rear timing chain components requires the engine be removed from the truck.
