[Chopper Dropper]

Originally Posted by Roo

Generally speaking:

Gears whine
Cause: lower finish surface standard, spalled surface, chipped tooth; after changing fluid, which flushes some debris, mating surfaces will "re-acclimate" and only get noisier over a long time. I've seen heavy equipment drivetrain gears with chipped teeth and spalled surfaces that worked noisely for hundreds of hours of continued us (still fully functional)

Bearings rumble/roar
Cause: spalling roller elements, get worse rapidly as damage increases exponentially

In either case, continuing to operate the transmission will potentially create new debris that can damage other parts (that's why helicopters have chip detectors in their gearboxes; chip-detected, land soon)

We're approaching the off-season, good time for a tranny refresh as noted by others.

Had a chip light come on whilst ferrying between spray jobs in a Bell 47 G 3B1 was at 200 ft or so, did not autorotate but landed pretty damn quick. Helicopter trailered to maintenance.