Workinonit,
Main bearings can make a deep 'thunk' under load and rod bearings can make a 'knock' when not under load and wrist pins can make a light metallic clicking sound all the time. If your noise is only evident when in fourth under a load I would think main bearings.
The main bearing knock shouldn't have come on overnight though. Vibration dampers can make a clunky noise too as well as a failing generator.
If you can really only hear it under a heavy load, it tends to make one think of a internal engine component, but the 'overnight' appearance clouds that theory.
Look around, wiggle stuff and listen to the truck as it idles. You might find something loose...rotten or loose motor mounts might be letting the engine rock over under load and rattle against something.
There are all kinds of possibilities, dig around.
Stuart
