I have a subwoofer that pops when not receiving a signal. The subwoofer works well when receiving a signal from the receiver. But when the receiver is off and the subwoofer in on/stand by it makes a popping sound periodically.
Disconnect the subwoofer input cable from the sub. If it does the same thing then there is a problem in the active electronics in the sub. Could be a bad capacitor but overall not a user repair. Contact the manufacturer.