Diagnosis
Likely a damaged driver (speaker unit) or something loose inside the casing. The passive radiators on the sides can also rattle if debris is trapped against them.
How to Fix It
Inspect the side passive radiators. Ensure nothing is stuck to the rubber surrounds, like a grain of sand or a hair. Gently wipe them clean. Check the fabric grille for any objects pressed against it. Play audio at low volume and gently press on the grille and sides to see if the rattle stops.
If That Didn't Work
The 2-inch full-range driver's voice coil is probably damaged from over-excursion. This happens from prolonged maximum volume with heavy bass tracks. The driver needs replacement. Contact Bose support.
Heads Up
The Flex has a built-in limiter to prevent damage. Pushing it constantly at max volume for extended periods can still overheat and damage the driver. Distortion is a warning sign—turn it down.