Sounds of unknown things present the narrator with a problem. You can't describe a sound accurately. Indeed, it is almost impossible to go beyond very basic onomatopoeia. So the narrator has to decide what general category of noise it is and its volume.
Is it sharp (technically, a "report") like the crack of a distant rifle? Is it a dull thud? Is it continuous? If so does it rattle, wail, drone, grind, etc?
How loud is it really? And how loud does it seem? (authors often get this wrong; for instance, your clothes make loads of noise as you move about but you don't hear it unless you're trying to be silent in a very quiet place).
How near or far is it really? How near or far does it seem?
Does it echo or resonate?
Answer some of these questions and you will be near enough without being vague, and why? Because the reader won't bother to imagine the sound until he knows what made it and how. Once he knows this, he will remember the sound at the right point in the narrative.
As he crawled through the darkness of the cave, Greg thought he heard, and became increasingly convinced he could hear, a regular sound ahead of him. Not a footfall, unless something was really dragging its feet; not breathing, as from time to time it seemed to sigh and stop.
He knew from bitter experience how your imagination could run you ragged into fear, and how panic now would be more likely to kill him than a thousand imagined terrors. And it would be the long, slow death of thirst and starvation, lost and hopeless in the tunnels.
As he advanced, and the sound grew louder, though no more distinct, he fell to concentrating on the sound itself instead of trying to ignore it. Pictures formed in the dark in front of him of vague scraping, small stones sliding, images of sandpaper sounds. He clamped his eyes shut and fought to concentrate on the shape of the tunnel and found the narrow ledge, passed two openings and entered a third, larger opening.
With his eyes open he began to think he could see light ahead, though he knew this was impossible. He had another mile to go before the entrance would reveal itself suddenly after a short elbow of tunnel. Now that he could walk upright, the sound seemed to diminish, and for a short while he didn't hear it at all - or didn't realize he heard it.
Remembering, he stopped to listen. It was surely closer now.
With the familiar gravel crunch under his feet, Greg at last began to relax, and breathe normally. He felt around inside his jacket for some chocolate and munched on it as he walked casually along the familiar tunnel in perfect darkness, turning the last corner from memory alone.
The wooden door in the barrier across the tunnel entrance had been left open, and was bouncing gently against the gravel floor in the light breeze. As he passed the door, he barely noticed the sound at all.