any ideas?

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. 

Equipment buzzing - taking a stab at this in the dark, the amplifier in your sub, when incoming AC encounters first a rectifier (diode/choke/etc) then capacitors to smooth out the DC power, If one or more of the caps in the power supply is coming close to the end of its service life, a weak power supply will cause your equipment to buzz.  If you're willing, unplug your sub, take off the plate amp off the cabinet and look at the caps, and if they are say bulging or even leaking, well then, that's likely your culprit.  Call the manufacturer, and find out who is the local authorized repair shop, and take it to them. 

If you added another piece of equipment or appliance on the same power circuit it could be a ground loop.  Try a cheep three prong adapter from the hardware store and see if that stops the hum.  Good luck and cheers

I am stating what is obvious.  If it buzzes when not playing anything, it's the electronics inside that is causing it.  Could be a ground loop or decaying parts in the sub amp.