Loud metal grinding sound when going uphill

My car is about 140K miles. It is a 2002 Subaru Forester.
For about one month now, my car has this loud metal grinding sound from the rear of the car when I try to climb a pretty big hill. (The grinding sound is rhythmic, it is more like Click Click Click, but loud and metallic)
After the climbing, the noise will slowly goes away.
The noise never more than 10-15 seconds.
The car functioned normally during all other driving but climbing this big hill. ( which I need go to through it at least 3 times a week)

Any one know what could cause this?
Thank you so much.

My guess is a CV joint.

Could be the front to rear torque split differential. The Subaru AWD system seems to require some very specific maintenance. If no one here is familiar with the system you might search for a Subaru forum.