I am pretty sure that the noise is coming from a dry bearing in one or more of the pulleys/tensioners for the serpentine belt. I had the exact same problem with my '02 Outback when it reached 8 years/100k miles. If one of these pulleys or tensioners seizes up, it could lead to more expensive repairs, so I would suggest getting it looked at a.s.a.p.
Raj suggested that the noise could be coming from a timing belt tensioner, which is also possible if this car has a 4 cylinder engine (the 6 cylinder uses a timing chain). Has the timing belt ever been replaced? If so, was the tensioner also replaced?
If a pulley or tensioner for the timing belt seizes up, it will cause extremely expensive engine damage, so although I think it is more likely that the problem is related to the serpentine belt’s pulleys/tensioners, the possibility that it could be related to the timing belt’s pulleys/tensioners makes this something that needs to be attended to right away.