Hi and thanks for using 2CarPros.com.
The tension-er should not be rattling. You should be able to move it, but it should require a lot of force to do it. Take a look through this link and see if it is helpful.
https://www.2carpros.com/articles/how-a-serpentine-belt-tensioner-works
Here is a link that shows how to (in general) replace one:
https://www.2carpros.com/articles/replace-serpentine-belt
With that in mind, either you got a bad tension-er, it is incorrectly installed, the belt routing is incorrect, or they sold you a belt that is slightly too long.
Check for those types of issues and let me know what you find or if you have other questions.
Take care,
Joe
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Monday, October 22nd, 2018 AT 5:45 PM