Clattering sound from bottom end
 

89 W150 318/727
At idle the bottom end sounds like the clatter of a diesel engine. It goes away when I give it gas. Wondering what you folks think it might be?
For the record: Fresh oil (60 psi when cruising and 25 psi at idle, vacuum gauge reads 18.5 at idle, and all cylinders have about 120 psi compression.

You could have a rod bearing or two going bad or a thrust bearing bad alowing crank to move forward and back making things come in contact with each other.

Other than being a mess, there isn't really a reason why I couldn't replace the thrust and rod bearings while the engine is still in, right? My truck is 4wd and has about a 3 inch suspension lift so I should have plenty of room as well.

Providing the crankshaft is ok and not scared or you haven’t spun a bearing you can replace the rod and main bearings in the truck, I’ve done it before.
The rods are fairly easy but the mains will need to be rolled into place, that is the upper bearing. You pull the main caps and let it hang on the timing chain torque converter. Sometimes the upper main will move when you rotate the crankshaft but most you will need to be pushed around in the direction of the tab that holds them in place.

before I would tear into it I would make sure to verify that it was the actual problem, with it running making the noise and the truck in park and park brake on get under it and try to isolate the noise torque converter bolts hitting the block or tins, timing chain or whatever it is.

good luck

Sounds like you just need to open it up and check to see whats up and go from there.... Keep us posted.... Hope its a simple fix for you... Cheers