what sound does the TT hear on his descent into the well?

The TT hears a sound that goes thud, thud, thud.

Sitting by the side of these wells, and peering down into the shafted darkness, I could see no gleam of water, nor could I start any reflection with a lighted match. But in all of them I heard a certain sound: a thud-thud-thud, like the beating of some big engine; and I discovered, from the flaring of my matches, that a steady current of air set down the shafts. Further, I threw a scrap of paper into the throat of one, and, instead of fluttering slowly down, it was at once sucked swiftly out of sight.