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If you take off the nut (7), washer (6) and pull off the pulley (don't lose the key, that half circle thing). If you can replicate the same squeal, then it's the generator bearing is likely failing.
If the squeal goes away, it's POSSIBLE that the pulley is vibrating or moving slightly on the shaft. Reassembly and see if it goes away.
My money is on an internal bearing though, which likely means a generator rebuild. (One of those things, you'll want to rebuild it BEFORE it fails, or it'll get far more expensive!)
BTW which generator do you have? The parts manual Vol 1 shows internal parts for all of the various alternators and generators, including the bearings. Likely not enough to actually rebuild it yourself, but it would show what you might be in for.