Termites, carpenter ants, wood boring beetles, bees or yellowjackets in the wall are among the insect possibilities.

If you suspect any of these critters, a professional service needs to be contacted.

Rodents are a common source of wall noise.

Bees, wasps, or hornets would likely make a low level hum or buzz; critters would be more likely to make something like scratching noises in the walls, or maybe you have a poltergeist :-)