CS Sub 10" Boston Acoustics is "Wood-pecking"

I have had my CS Sub 10" by Boston Acoustics for about two years. Around November 2015 it started making noises not sent by my receiver. I describe the artifact as the sound a woodpecker makes when pecking wood. I tested to find the cause by trying a different outlet, trying a different sub cable, I tried running the sub at line level, and I have tried having no connection to the receiver at all. All of these test produced no change in the artifacts. These artifacts come and go but always happen many times an hour.