Strange noise under the hood

Check the tensioner and idler pulleys. Remove the serpentine belt and check the pulleys. If you can wiggle them off-axis, they need to be repaired or replaced. Also rotate them by hand... if they feel "grainy" or "crunchy", they need to be repaired or replaced. Also check the other pulleys, such as on the A/C compressor, for the same play... and obviously make sure all bolts are tight (could be something else loose like a heat shield or bracket).