If you're lucky it might just be pinion nuts backing off. If not too late it might be possible to tighten them. The loud noise is caused by the helix angle pulling the pinion into tighter mesh with the ring gear.
It could be something else, but that would be my first investigation.
If it were the transmission it would sound different in different gears. That said, it may sound different in each gear due to heavier loading with lower gears.
Mart.