Ok, I was going to keep my mouth shut, but I can't resist.
Here are the things that I've heard sound like rod knock at one time or another.
- Fried rod bearings. (duh...)
- Dead piston
- Dead valve
- Dead valve guide
- Loose crap in the head - ball bearing, nut, bolt, all kinds of crap that people have forgotten about while reassembling
- Loose crap in the block (oilpan, anyway) - same as the above note for the head
- Loose crap behind the timing belt cover - this was relatively bad, the cam gears exploded shortly afterwards, pretty certain the noise was a piece of them rattling around just before they completely self destructed
- Loose crap in a cylinder. Too big to go through the valves, was rattling about, sometimes it would get caught between the piston and head / valve and make an ugly noise, sometimes it would fit and be quiet
- Valve shims out of spec
- Exhaust cam missing CPS tooth
- CPS missing tooth
- Broken flexplate on the automatic transmission
- Loose flywheel bolts
- Loose main pulley bolt
- Dead main pulley
- Dead main pulley key
- Noisy injectors
- Exhaust leak, especially on the EGR
- Dented oilpan hitting the crank
- Dead alternator
- Failing waterpump
- Dead clutch spring - one of them had self destructed, one big piece of it was left in the clutch plate, rattled nastily as the engine idled, got worse when engaging / disengaging the clutch
- Dead motor mount - entire engine would shake, causing a rattle in time with the RPMs
These are just the ones that I can remember off the top of my head. There are more. LOTS more.
SC's thread makes sense, after having exhausted that I would resort to pulling the engine and tearing it apart. Eventually I'd find the culprit.