Here's a list I made up of the common complaints I saw on an "unnamed" forum.
I Hear the Sound When...
Clutch pedal not pressed, or in neutral
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Replace the input shaft bearing.
Clutch pedal pressed halfway
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Replace the throwout bearing.
Clutch pedal fully pressed
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Replace the pilot bearing.
In all gears, regardless of clutch position
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Temporary: get better transmission fluid.
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Real Solution: transmission rebuild.
I Hear...
1st gear whining
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Replace the input shaft bearing.
Grinding when shifting
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Temporary Fix: shift more slowly and get better transmission fluid.
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Real Solution: replace the synchros, shift fork, and/or clutch.
Notchy shifting
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Temporary: get better transmission fluid.
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Real Solution: replace bearings and/or synchros.