I had a squeak at idle and realized it was the belt around the alternator. I have had a mechanic tighten it for me. It was fine for a week then started squeeking again. He came with a big bar for leverage and tightened it again. Its been about 2 weeks and its loose again and squeeking. Why does it keep loosening and what do i do? Btw its a 1990 Mercruiser 454 Mag 7.4 L