any ideas?

If you can download and play a low frequency bass cadence of low frequency test tones their continuous play length should allow you to more easily locate the source of the buzz.

Once located, contacting the manufacturer should offer definitive suggestions or options.

Good luck with it.  

Carry the sub to a different room on a different circuit and plug it in. See if the buzz goes away.

If the buzz is 60 HZ it might be the power supply transformer mechanically buzzing. I'd call Rythmic and ask. I have one and they are good folks to do business with.

If it s.not the speaker, that leaves the box and the electronics. My first instinct is to look for a compelling ent that has come slightly loose from its mounting. Either pull the amp ate or remove the woofer to get access and see what you can find, then hotglue, silicone, or tywrap the offending piece(s) back in place. Don't overlook the internal wiring, either. But I'd guess the power transformer or another component like a large capacitor leaning onto it.

Subwoofers put lots of mechanical stress on the cabinet and internal component. Over time mechanically connected (screwed, bolted or riveted) components may loosen.  Bonded components (soldered, adhesive, or solvent bonded) may crack from physical stress or detach based simply on aging. Speaker suspensions or coils may also produce mechanical vibration as they age.  You need open the cover and see if you can identify the source of the vibration with your ears and by touch and feel.  Use caution and take proper electrical safety precautions.  If you are not comfortable, find a local electrical repair shop. They are becoming rare, but they are still around.