Based on the work that was done prior to the problem, the first thing I suggest is to check the condition of the "new" belt. What you are describing sounds like it may be damaged and a small portion of it is whipping around and hitting things. Also, there is only one belt, a serpentine belt. I attached a pic below and circled the belt. I highlighted the alternator that was replaced.
Inspect the belt for fraying or damage. Also, the next time you hear the noise, slightly rev the engine to see if the sound speeds up.
Let me know what you find.
See pic below.
Tuesday, April 13th, 2021 AT 7:38 PM