A slight rattle started to be heard from the engine bay several months prior to replacement being needed. I took it to the dealer a week later when. They had availability. They said they couldn’t hear it and sent me in my way. Sounded like a really loud PCV valve if you recall or know what those used to sound like when going bad. It worsened over the next few months enough so that it became very obvious and it noticeably followed rpm. I took it to a different dealership and they diagnosed a bad bearing on the idler/tension pulley and replaced the pulley at my expense. Not covered under warranty.