Rythmic thumping coming from sub

Greetings,
I've had a rhythmic thumping coming from my sub (PSW125) for about a month now. It happens even when my receiver is off. I'm looking for advice on how to troubleshoot or isolate the issue so I can be certain it is the sub. Thanks for any insight.
Receiver: Yamaha RX-V479
Speakers: Polk Audio Blackstone TL250 5-piece home theater speaker system
Sub: Polk Audio PSW125 (BK) 12" 150-watt Powered Subwoofer
I've had a rhythmic thumping coming from my sub (PSW125) for about a month now. It happens even when my receiver is off. I'm looking for advice on how to troubleshoot or isolate the issue so I can be certain it is the sub. Thanks for any insight.
Receiver: Yamaha RX-V479
Speakers: Polk Audio Blackstone TL250 5-piece home theater speaker system
Sub: Polk Audio PSW125 (BK) 12" 150-watt Powered Subwoofer
Comments
Rhythmic thumping with receiver OFF = Not Good.
So far, so good.
Disconnect wires from the back of the sub. What do you have now (with the sub still turned on) ?
Here to pick your brain & steal your cookies
Shifting to Plan B+
2 Channel Thorens TD 318 Grado ZF1, SACD/CD Marantz 8260, Soundstream/Krell DAC1, Audio Mirror PP1, Odyssey Stratos, ADS L-1290, ICs-DIY Twisted , Speaker-Raymond Cable
Like mrbigbluelight said, start unplugging signal wires and see if the thumping stops. If not, it's either wireless signal interference or there is a bad ground.
If the Subwoofer is out of the sub and the subwoofer is on and it still thumps then it is definitely the amplifier in the subwoofer.
The amplifier has a 3 year warranty on it.
You can message me directly and I will take care of that for you.