Nameless Things

[Kransha]

This forum seems appropriate for the following query.

In TTT, Chapter - The White Rider, Gandalf is telling the Fangorn trio of his plight with the Balrog and says, in the process...

"Far, far below the deepest delvings of the Dwarves, the world is gnawed by nameless things."

Of course, this could simply be some metaphor or flourish of speech from Gandalf, but later he continues, saying...

"Even Sauron knows them not. They are older than he."

Well, that doesn't make a lot of sense considering Sauron is as old as Arda. This would suggest that there are some living things, possibly even Maiarn of a kind, living far below Khazad-dum, gnawing at the roots of Arda itself.

My question: Is this a metaphor/symbol/colorful addition to speech or could Gandalf actually be referring to "Nameless things" which he encountered while battling up the Endless Stair with Durin's Bane? If so, despite being nameles, what might these things be?