2009 Toyota Highlander, 165K, runs great! Except for noise in the engine bay, somewhere in the pulleys. Local mechanic replaced the pulley tensioner about 5 months ago, which did help, but then the same sound came back. I took it back to him, while the engine was running he showed me and I saw the 'new' tensioner jumping around, so he said it must be a defect because it shouldn't be jumping around like that. So, he ordered it again on warranty (OEM) and just charged me labor. The next day the sound came back! I looked at the tensioner he just replaced and it looked stable, not jumping around like he previously showed me. So i'm confused if this is really the root of the problem? He's gonna take another look this Wednesday and I just wanted to run this by you guys...

Also, I've noticed over the days this sound comes and goes, hot or cold engine, with or without A/C, it's not a consistent sound. Last night I started my car at 8PM with it cold and it was absolutely terrible sound, worse it's ever been, screeching to the high heavens! I put it in neutral to back down the driveway and the sound completely went away, put in drive to go and still quiet, so I dunno.

Thank you!