What structural problem usually indicates an active infestation of wood-destroying pests?

The presence of an active infestation of wood-destroying pests is often indicated by several structural problems, each providing evidence of different stages or effects of pest activity.

Visible decay on the surface of wood can suggest that wood-destroying insects, such as termites, are present and causing damage. This decay is a direct result of the breakdown of the wood's structure by the pests as they feed.

Mushy wood in certain areas is another critical indication. When wood becomes soft and weak, it often means that pests have been tunneling through it, compromising its integrity. This condition is typically associated with moisture problems as well, which can attract wood-destroying insects.

Insect activity noises in walls, such as buzzing or clicking sounds, can be a sign of active pest presence. Many wood-destroying insects communicate with one another, and these noises are indicative of that ongoing activity within the timber structure.

Thus, since the indicators of visible decay, mushy wood, and insect activity noises all highlight signs of an active infestation, it is correct to conclude that all mentioned problems can serve as evidence of wood-destroying pest presence.