any ideas?

Thanks all!  I took off the plate amp and the caps ARE all bulging and at least a couple of resistors are burnt looking as well.  Contacted Rythmik and in the process of purchasing a new amplifier.

If the noise is constant whether music plays or not, AND it’s not coming from the speaker... that eliminates pretty much everything.

It could possibly be a buzzing inductor, though that’s kind of reaching. By nature all inductors buzz so they’re potted in resin or glue to quiet them down. The dampening material can fail with age.

If that’s what it is, it’s completely harmless.

Power supply caps have degraded, or transformer dying. 

I second the above advice to contact Rythmik. That guy knows his products extremely well and their customer service is famously quite good.

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.

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

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. 

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. 

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.

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.

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

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.