If the S1 is vibrating/rumbling lightly at certain angles with any camera that requires BacPac mode and Balance Weight Cap such as: GoPro Hero 5 Black, Hero 6 Black, Hero (2018), Hero 7, etc. Please make sure the gimbal is calibrated first: https://removu.zendesk.com/hc/en-us/articles/115003982447-When-and-how-do-I-calibrate-my-S1-Gimbal-
If the gimbal is still vibrating/rumbling, chances are the mounting point for either the S1 or the frame housing is loose. Normally the frame housing needs to be replaced first. If that didn't help, the unit needs to be repaired/replaced by support.
As a temporary workaround, please place a soft object (like a thick rubber band) in between the mounting points (see image below where the red arrow indicates) and see if the issue go away. Make sure the object is not in the way with any moving parts.