Diagnosis
The soundbar has integrated subwoofers, not a separate unit. Distortion or rattling means a driver is damaged or something is loose inside the cabinet.
How to Fix It
First, lower the bass level using the remote to see if the distortion stops at lower volumes. If it does, the driver is physically damaged from over-excursion. Play a bass-heavy track and gently press on the grille over each side-firing woofer. If the rattle changes, that driver is faulty.
If That Didn't Work
Check for foreign objects. Debris like a small toy or paper clip can fall into the ports on the ends of the soundbar, causing a rattle. Inspect carefully.
Heads Up
Do not block the side-firing speaker ports. This can cause overheating and damage the drivers. Keep at least 4 inches of clearance on each side.