Because I don't know the whole story yet. From this question alone above, then, also did not write which dynasty? So I can only estimate. What is this thing that makes a sound?
Could it be an animal or an echo of a sound? Because the speed of sound propagation is much faster in the underground than in the top. Because of the air, sound travels more or less slowly, but underground the sound travels faster.
So let's assume a few things in this case. First, it's a strange sound coming from an animal. We all know that many animals, it is possible to burrow into the earth to live. So it's normal for it to make strange noises inside the earth.
Secondly, it is the echo of some kind of sound. Sometimes the sound can be echoed back and we can hear the echo. It's possible that the sound is being reflected back to us in some way, and we are hearing it.
We also hear so-called echoes in our daily lives. Let's say we go up a mountain and shout at it. Then it doesn't take long before you gradually hear the peak return to you. Then you can hear your own echo again! In fact, this kind of are similar to more.
Summary of the strange sounds coming out of the words is likely to be some kind of animal is drilling soil type, he lives in the soil in the land under the sound produced by the echo. This kind is more common in our life.