Saruman.
And didn't Gimli look for boot tracks, but can't remember if he ever found them. And didn't Aragorn say something like they didn't matter? And Legolas would have spotted that it was a phantom, so most likely it was Saruman out for a stroll, and having come across three hunters in the woods - no hobbits, no Rings, etc - he left as the three were of no concern.
And just what was that small rumbling in Fangorn?!?
Or maybe it was Tom Bombadil's shyer brother...
