This is because the old belt was slipping and has polished the pulleys or "glazed" them. So before you replace that noisy belt scrub all of the pulleys well with a scotch brite pad, or a good wire brush to remove the glaze and rough up the surface. Clean them off with brake cleaner afterward. Belts work by friction so if the surface is polished too much the belt will slip. If you have already put on a new belt and it is making noise, -sorry you must buy a new one again. Slipping on the pulleys will glaze a new belt fast which ruins it. Some people say you can apply brake cleaner etc to a new belt to fix it but that may not work well. If it slips it will just mess up your newly cleaned pulleys and you will have to scrub them up all over again. So do the job right and rough up the pulleys AND replace the belt at the same time.