OK MY bet is its NOT bearings, its your rear axle and its about to fail on you
weak links on 450's and like rear tubed axle hondas is the left side wheel bearinsg/sleas, go bad, water gets in, and rusts splines off axle, then sooner or later the rusty splines strip and NO more rear drive
the fact its NOT making the sound with NO weight on the tires, says this to me
GOOD news is, its an easy job , bad news is a rear axle isn;t cheap
check your left rear wheel for any play, if there is, that is my best bet!
get soem new seals/wheel bearings(about 35 bucks), and well, your going to need a new rear axle, abouta 175
the flaw in design is, left side only has one bearing, right side has two
so the left side fails faster!
rather common deal here!
I think I measured this as it was still a good axle,
again sinple job, just basic hand tools here
pull your rear axle and measure the splines, and look to see if there bad