hey Huy
maybe i can help u on this one

i had this happen to my car for the last 2 weeks or so, and yesterday i brought it in to city subaru and they automaticately knew wat was wrong. The thrust bearing!!!!
they ripped it all apart and replaced a new one under warrenty and now its GREAT

things i noticed when the clutch was squeaking

1. the noise is from the engine bay and not the pedal
2. it happens only after a decent drive, time wise, or in hot weather, but never when the car is cold or left overnight
3. the clutch pedal feels course, hard to explain but like as if steppin on a bottle of sand, u could definatly feel the difference between the good and bad days.

other than that it engages just fine

get it checked out if u still have warrenty cos i think its a pretty common problem