An indoor unit that rattles may have loose panels, debris, or a failing fan assembly

https://localcoolinghelp727.zenbloomer.com/posts/why-your-ac-has-weak-airflow-and-how-to-diagnose-it

An indoor unit that rattles may have loose panels, debris, or a failing fan assembly. Turn it off and tighten only visible loose screws if it is safe to do so.