Why would my 2000 Mazda 626 start knocking after replacing the alternator? It did not knock with the old alternator.
Make sure your alternator and belt are properly tightened. A loose belt can make all kinds of noises, but mostly a loose alternator, under tension form the belt, will be spinning on an angle, and that's not good.
Any play in the tension of your belt or loose pulley bearings can cause odd noises. Sometimes shops replace the internals of the alternator and rebuild it. If not done properly they can make all kinds of noises. It's best to determine where exactly the knock is coming from. You should listen as closesly as possible to determine where it's coming from. You can use your hands or a screw driver to feel and listen to parts of the engine by touching it to different components. DO NOT put it too close to anything moving. If this knocking just happened after your alternator was replaced though, it's probably just something loose that they didn't tighten or the alternator is bad.