Occasional squealing sound when clutch disengaged

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:

  1. 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.

  2. 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.

  3. 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).

Good observation and smart decision.

Nice work That’s good life for a clutch.
If it were mine, I’d just keep driving it until it begins to show signs of slippage. Chances are good that your friend’s attempts to drive it resulted in a bit of glaze on the clutch plate’s frictional surface. Or perhaps the flywheel frictional surface.
If your friends notice, just tell them "I’m sorry, I had pork & beans for lunch… ".

Have you checked your tire wear?

When you press on the clutch pedal a bearing gets put to work that ordinarily just sits idle, called the clutch throw-out bearing. It might be that doing the squealing.

Good idea with that symptom to check the trans fluid level and if there’s a rear differential, check that fluid level too. Tire problems are the more common cause.

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