2005 Subaru Outback. Occasionally the car will produce a squealing sound when I disengage the clutch to shift up (never occurs when down shifting). Does not occur all the time and when it does it is only for the first 10 to 20 minutes after starting to drive. Is much more likely to happen first thing in the morning when the car is cold. The only way I have of replicating the problem is doing a hard acceleration in a low gear and then up shifting quickly (e.g., trying to pull out into fast moving traffic) and this only works if the squealing has already occurred.
Things that may or may not be relevant:
One of the serpentine belts has a lot of tiny cracks in it. Hopeful theory is that is the problem. I should obviously just replace it anyways.
Car has 175k miles (bought it at around 60k) and I have not had to replace the clutch yet, despite a bit of abuse by a friend who claimed he knew how to drive a manual transmission. No signs of clutch slippage though.
There is a low volume humming whose pitch is relative to wheel speed, not engine speed (thought it might be a wheel bearing but volume does not change with steering angle).