Sonus Faber Gravis B1 subwoofer

I have recently purchased a used Sonus Faber Gravis B1 subwoofer to go with my Wilson Benesch Arc standmounts.

There is audible hum from the sub which is audible from 6 feet or more away...could anyone who owns one of these subs tell me whether this is normal for this sub?

I have tried powering off everything else in the room in case it is picking up interference but that makes no difference.

As an aside, apart from the hum I am loving the sub!
Check if it’s a ground loop (use an outlet on a different circuit). 
Since even $100 subwoofers don’t have this issue, I highly doubt it’s a native issue with the sub.
Thanks for the advice mzkmxcv, I'll give that a try.