Help pleez!

My ute is running a 1500 flywheel that has been lightened by about 7 pound from memory. It had a sunny 5-speed behind it which sadly blew up recently. The noise it made as it was dying was the sound of the spiggott bearing getting shredded after inspection. Almost like a sound of something breaking under load, an electrical sparking sound perhaps, tinny sounds maybe like gravel hitting the undercarriage.

Anyway, I replaced with the original 4-speed box and low and behold I get the same noise - intermittently.

We pullled the box out again, it's got new seals and bearings, measured up everything against 4-speed flywheel spec's. The clutch is OK, everything's fine - so WHAT is wrong? Do I have to change back to a 4-speed flywheel or could it be something else, maybe not even gearbox related???

Thanks