engine "noise" at 2000 rpm only?

[fixkick]

crack fix:
some might just do that (wait) , drive it, until the MPG goes nuts.
yes, the welding of cast iron preheated first is hard to do.

that rattle sounds to be , like:
1: CAT shield. loose or cracked, (pry it)
2: or loose header flange.(donut long header) or flat gasket and bolts short version.
3: main intake manifold bottom mount to top rear exhaust manifold mount loose, use levers to find it, a 2x4 board levered with say 20 lbs force, on many places, exhaust pipe to rear to.
and do be sure to stay away from fan and belts.

i don't think its actual engine, it self. based off video sound track.
good luck @