It's the belt tensioner. I had this problem from day two and Toyota gave me the run around about it since. It hasn't come back until recently from 6-8 months ago. I thought the belt healed itself. Crickets under the Hood
, I told Toyota. They said, I was crazy. Belt tensioner sprays are crap and shouldn't even be used on a serpentine belts like ours even knowing that there is a problem with the tensioner. Believe me I just tried it many times. My belt looks a little worn so I'm going to replace it stock, try the gator belt, or go AEM
(even though I don't want to because I'll know what will happen). If under warranty have Toyota fix it. If not do a search here and good luck.