Sounds like it needs to be replaced. May be time to change the serpentine belt also. When the belt is off check all the other pulleys to make sure they are not making noise, and are in good shape. May just want to replace the idler pulleys anyway since you have the belt off.
Its most likely time to replace it. If belt is older replace it too. It may be getting worn because of it and actually not a bad idea to replace the other tensioners/idler pulleys too as I have replaced a failed one, put on new belt just to have another go out completely and chew the new belt up.
I have replaced all the pulleys and belt and the power steering pump to and the AC compressor have gone through 5 Tensioners I just can't figure it out
Few causes are out of alignment, extra strain, either belt or tensioner or pulleys and can be because the bolt threads holding it on are worn allowing it to move, vibrate, needs a re-tap & new larger bolt. After-market belts may also be way these issues pop up, I do like using OEM when possible but with ford its more precise as its designed to be and after-markets fail to bring this and its quality, try OEM belt. Be sure its the correct belt length. Install it to the correct torque specs, to tight can damage it. Is water leaking on the front or the motor. Be sure all pulleys. rollers are moving easily, no drag. May not be a bad Idea to replace them all. there are some complaints about idler pulleys and such on record about the Focus and you may should research this and see if any recalls to correct have been missed and you may need to purchase these refined new parts to install next time. Also if there appears to be on going issues then Look into a complete pulley & idler & tensioners & belts system from either another car or from a company that is designing these systems to resolve known issues as a complete repair (possible).
When looking at the belt threw the pulleys, does is look like it is all lined up? Is the pulley on the belt tensioner grooved for the serpentine belt, or is it a smooth pulley that rides on the back side of the belt. And is there play in the spring in the belt tensioner so it can move up and down with the belt? Just wondering if there is an alignment issue.
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