Sorry to say mate, it sound like something serious inside the engine. If first mechanic said main pulley then ask him to guarantee that and get it changed.
For sure its not normal, so whatever you do get it fixed or else wait for big bang which might cost a fortune to fix. My friend car serpentine belt broke and snapped the radiator shroud, radiator fan blade and made big dent in radiator. Total repair cost was 1200 dhs for 80 dhs serpentine belt that didnt changed by him in time.