Had a work truck here, same year, same engine making that noise. Ended up being the manifold.
and what a bitch to get that off. Snapped 1/2 the bolts.
It wouldn't make any noise once it got to speed, but would make a horrible ticking sound during acceleration, or up hills.
270,000 miles on that truck, and its been abused. Still runs like a champ, and still the original transmission.