New alternator is vibrating and knocking against bracket?

If a new alternator is vibrating and knocking against the bracket, it may be improperly mounted or misaligned. Check the mounting bolts to ensure they are tight and that the alternator is seated correctly in its bracket. Additionally, inspect the belt tension; if the belt is too tight or too loose, it can cause vibrations. If the issue persists, consider examining the alternator itself for any manufacturing defects.