Transfer case noise?

[georgeinmd]

Wonder if anyone's experienced this:

 

'99 Forester MT 130K miles -- there's a whirring or clicking (depending on speed) coming from the transaxle. Listening with the shift boots removed, it seems to be from the transfer case/center diff./tail shaft housing. Frequency directly proportional to vehicle speed, not dependent on gear selected, but much more noticable on deceleration than acceleration. Gear oil looks fine, and tail shaft has no unusual play or looseness. Been getting louder for about a month. Any ideas??? Thanx -- George