Hollow things are common, and of interest to many animals. If I thump a log and it makes a noise like it has a hollow space (low tones), then it may contain an animal nest or a beehive & honey, or it may be something I could use as a box or basket or shelter.

Scratch a thin pointy branch across e rock -> sharp high noise.

Thump a round club/log against a rock -> dull bump noise