Diagnosis
Ground loop or electrical interference. The subwoofer's power supply is picking up noise from your home's wiring or other devices.
How to Fix It
First, plug the subwoofer's power adapter into a different wall outlet. Use an outlet on a different circuit than your TV and soundbar. Ensure the sub's power cable is not bundled with other power cords or signal cables.
If That Didn't Work
The wireless connection between the soundbar and subwoofer may be picking up interference from a router or microwave. Press and hold the 'Wireless' button on the subwoofer's back for 5 seconds to re-pair it with the soundbar. Do this with potential sources of interference turned off.
Heads Up
Do not use a 'ground lift' adapter on the subwoofer's plug. That is a safety hazard. If the hum persists, the subwoofer's internal amplifier may be failing.