Would rear drum breaks on a cb550 make any noise....when the bike is in motion but the brakes are not being applied?
HAs it been sitting for a while? Gunk build up is now sluicing around with the brake shoes.
Back off the adjuster as much as is safe and see the difference.
In any event, I vote for simple dirty build up of brake shoe dust and gunk. Disassemble and while standing outside, blow it out (hub and backing plate) with air. Do not do it inside. Maybe wear a dust mask.
Wipe the drum off with alcohol or such. Don't touch the shoes with anything, fingers especially.
Just an FYI: I've never seen a Honda MC shoe wear down to the shoe. Just too much material on them. So that's not likely.
Could be bearings too.