Squealing like a bad belt? It could very well be a bad belt. The load at lower RPM could be the cause. Try putting some dish soap on the inside of the belts and see if the sound goes away, if it does then you just need to figure out precisely which and replace the offending belt.
If it's making a swooshing and possibly lightly grinding (like sandpaper being run over something) then it's likely a bad throwout bearing or pilot bearing. Does the clutch still grab fine?
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