Thanks for the info, wmax. I took the alternator apart this morning and although I couldn't quite tell what was catching, I guessed that it was the inner edges of the fan catching on the inner part of the housing. I bent some of the fins that were especially close to the center, reassembled, and all seems to be well!
The guy I bought it from said he had tested it - and I believe him - I just don't know how it was working if the fins were catching like that... Maybe once it was powered the fins just scraped past. Anyway, thanks both of you for the help!
