I have a 2013 Subaru outback 2.5 4cyl. I replaced the serpentine belt about a month ago because it was squealing. The new belt stopped the noise for about a month and it returned. I thought maybe it was the automatic tensioner being worn, but would like to get someone else's take before I just start taking things apart.
What you want to look for is a pulley that is dragging. Take the belt loose and look at your idler, does it spin freely if so it is not that. It could be the water pump or the alternator. Look for loose bearing by seeing if they wiggle side to side.