I had noise coming from my muffler. Was told it was a little crack and fixed that. Drove that day and heard lots of noise reversing and while ac turned on . Went back and mechanic said I had to change both belts(mine you serpentine belt was changed not too long ago). Sound was still there, took it back and then he replaced the tentioner. After replacing that; the sound was better, but gradually came back about 2 days driving I'll turn the AC and also putting it in reverse that sound would come. Now he's saying he only sees the tentioner.. right now I don't want him to go anything else; I'm ready to do it myself. Any help will be much grateful.