My son's digger just started making this crunching / grinding sounds. You can hear it more in reverse than forward. The truck seems a good bit slower as well.

Here's what we already did:
Checked to see if there was anything stuck between a-arm and axle.
Checked wheels for rounded hex.
checked axles for chewed up splines.
Took the motor out and cleaned/reoiled it.
Tore transmissions apart to see if there were any broken gears.

Anything else we can look at? He only got the truck this past Christmas, is there any warranty that might cover this?