Obstruction

• Subwoofers push air away from them when functioning, which is capable of moving a small or properly placed object. If you have something leaning against or sitting on top of the subwoofers, you may be hearing that object hitting against your woofer or blowing back and forth in the air.
  
  

Wiring

• Loose or bad wires can cause an intermittent snapping sound in your subwoofer, which can be remedied by tightening or replacing them.
  
  

Poor Craftsmanship

• You may have a loose or missing screw on the subwoofer itself that is causing the speaker to slap against the enclosure. You also possibly may have a cracked or poorly built enclosure for your subwoofer.
  
  

Blown Subwoofer

• If the snapping noise still remains after checking for other possibilities, or there is an obvious tear in the subwoofer, you will need to have it replaced.