The correct belt is 10mm,the 9.5mm belts were dropped many years ago.
Belt tightness,one inch deflection under moderate pressure.
Alternators can make noise under heavy load,humming or type of squealing.
Warning on most alternators,Battery must be fully charged before running.
Why?Alternator may over heat,was not designed to run at max.for a long period of time.
A voltage/amp test of alternator is needed possible also of battery.
Remember new does not mean good.Testing is required.