Diagnosis
Yeah, this one's common. It's not normal, but it's a frequent gripe. The hum is electrical noise, usually a ground loop or interference coming through the power supply.
How to Fix It
First, try plugging the subwoofer's power cord directly into a different wall outlet, not a power strip or surge protector. If that doesn't silence it, unplug any RCA or HDMI cables from the soundbar except for the power cord, then check the sub again. Sometimes the audio cable itself acts as an antenna for interference.
If That Didn't Work
If the hum changes pitch or volume when you turn lights or appliances on/off, the issue is in your home's wiring. You might need a cheater plug (a 3-to-2 prong AC adapter) for the subwoofer to break the ground loop, but use that as a last resort.
Heads Up
A very faint hum with your ear right up to it might be unavoidable. A loud, audible hum from across the room is the problem you're fixing.