Could it be an echo of an animal or a sound? Because if you talk underground, the speed of this sound is much faster than above. Because there is air, sound travels more or less slowly, but it travels faster underground.
So we assume several situations in this situation. First of all, this is a strange sound made by animals. We all know many animals. They can live in the soil. So it is normal for it to make strange noises in the soil.
The second word is the echo of some kind of sound. Sometimes sounds can be echoed, and we can hear echoes. Then it is very likely that some kind of sound reflected back in some way, and then let us hear such a sound.
We also hear the so-called echo in our daily life. For example, let's climb a mountain and shout at it. Then before long, you will gradually hear the voice of the mountain returning to you. Then I can hear my reply again! In fact, there are many similar ones.
To sum up, the strange sound is probably the echo of an animal that lives in the soil and makes sounds under the ground. This is very common in our life.