המועדפים של wormsy

What is this, it's driving me crazy?!
There was some kind of growling/rumbling/creaking coming from under the thick layer of leaves on the floor. It was coming from many different spots in that area, dozens of it right under that tree in the photo.
Once I found a lizard running away, but I just couldn't find anything else that would make such a sound.
Does anyone have an idea what it could be?