You'll need to drain the transmission to check this but check the inside of the axle gridle for marks. If to much pressure is put on the gears it will make a sound like yours.
Many noises can occur from the rear:
NOISE ACCEPTABILITY
Drive axles produce a certain amount of noise, as all gear-driven parts do. Some noise is acceptable and may be audible at certain speeds or under various driving conditions, such as a newly paved blacktop road. The slight noise is in no way detrimental to rear axle operation and may be considered normal.
With Traction-Lok limited slip differential axle, slight chatter noise on slow, tight turns after extended highway driving is considered acceptable and has no detrimental effect on the axle's locking function.
GEAR NOISE
Gear noise is the typical "howling" or "whining" of the ring gear and pinion due to an improper gear pattern, gear damage, or improper bearing preload. It can occur at various speeds and driving conditions, or it can be continuous.
CHUCKLE
Chuckle is a particular "rattling" noise that sounds like a stick against the spokes of a bicycle wheel. It occurs while decelerating from 40 mph and can be heard all the way to a stop. The frequency varies with the vehicle's speed.
KNOCK
Knock is very similar to chuckle, though it may be louder and occurs on acceleration or deceleration.
CLUNK
Clunk may be a metallic noise heard when the automatic transmission is engaged in REVERSE or DRIVE, or it may occur when the throttle is applied or released. It is caused by backlash somewhere in the driveline, or loose suspension components.
BEARING WHINE
Bearing whine is a high pitched sound similar to a whistle. It is usually caused by malfunctioning pinion bearings, which are operating at driveshaft speed. Bearing noise occurs at all driving speeds. This distinguishes it from gear whine, which usually comes and goes as speed changes.
BEARING RUMBLE
Bearing rumble sounds like marbles being tumbled. This condition is usually caused by a malfunctioning wheel bearing. The lower pitch is because the wheel bearing turns at only about one third of driveshaft speed. In addition, wheel bearing noise may be high pitched, similar to gear noise but will be evident in all four driving modes.
CHATTER ON CORNERING
Chattering noise when cornering is a condition where the whole rear end vibrates only when the vehicle is moving. The vibration is plainly felt as well as heard. In conventional axles, extra differential thrust washers cause a partial lockup condition which creates this chatter. Chatter noise on Traction-Lok axles can usually be traced to erratic movement between adjacent clutch plates and can be corrected with a lubricant change.
CLICK AT ENGAGEMENT
Click at engagement is a condition on axles of a slight noise, distinct from a clunk that happens in REVERSE or DRIVE engagement. It can be corrected by installing a slinger between the companion flange and front pinion bearing.
