Engine makes a noise.

I have the identical truck. They had defective exhaust manifolds and had sparkplugs which could loosen up. The metal in the exhaust gasket and the metal exhaust manifold start out with an initial noise which becomes less when the engine produces heat making the metal swell and close the gaps. As things get worse the gaps stay open.

The bolts that hold the exhaust manifolds on the engine will fail. They need to be drilled out of the heads and replaced as well as many of the manifolds warp or crack.

The repair takes skill or you will trash the cylinder head. A cheap price per side is at least $600 on one side to $400 for the other. The price difference is due to the amount of time difference needed to get at the parts. You will need a bolt kit for each side. Many mechanics use a 90 degree drill, a cable drill drive, or dremmel to remove the old broken bolts from the heads. Punch through a water or oil passage and the head is done. A heli-coil can only do so much.

I suggest doing a Web search for repairing or replacing the exhaust manifolds on this engine. You tube has several.on this subject. It may be easier to pull the engine and do the drilling and replacement of the exhaust.

Also be careful because there were 4 exhaust systems for the V-8 alone. There were 2 automatic transmissions in 1999 and each had a separate crossover for 2 wheel and 4 wheel drive. There was also a running change in wheel stud thickness depending on production date.

I hope this is helpful, there was a recall, but I did not drive much so my failure was years after the recall.

The IAC wouldn't cause a noise like you describe, and as for the spark plugs, it is possible but only if the plug broke off, or blew out of the engine. You say engine is running OK but a V8 can do that on 7 cylinders. Also you may be describing the beginning of a piston rod knock. You will need to check further. Good Luck Friend