The force of the belt being installed can often push a pulley out of alignment. When the system is relaxed, it might not be apparent. Start by checking all bearings in the pulleys and tensioner for any side to side play. Rotate the tensioner stop to stop while observing the tensioner body for movement. There should be none. Then, inspect the grooves in the tensioner pulley and idler pulleys for contamination or wear. If you are still having issues, conduct a noise test. With the vehicle running, spray water on the belt. If you hear a chirping noise, there is misalignment somewhere in the system. If the belt is squealing, where the noise goes away and returns quickly, there is an issue with tension and the tensioner should be replaced.