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OR..........cracked bracket? cracked lower hole in alt case? spacer missing at lower bolt hole?cracked/broken bolts? loose pulley? misaligned pulley on alt? damaged balancer on crankshaft?
Bad belt is my vote. Remove belt, inspect and replace it.
Spin alternator and see if it is obvious before tearing it down is my advice. Alternator is fairly easy to tear apart but the front nut can be a booger.
My 65 was making noise, took it to local starter alternator shop $50 new bearings & checked out.
I also agree with Door Gunner double check that mounting bracket, something looks loose.