I picked up this 2007 focus with 80,000 for my kid. Very nice one owner car, but a little on the loud side. After some research I decided to replace the tentioner and the idler pulley. I also found that some people thought it was the alternator, which is not the case. There are plenty of videos on how to actually replace these parts, so this is just the symptom and result... We also had some vibration issues, so I replaced all 3 mounts. I wish I had a video while driving, it's like a different car.