I had a thought that the words we tagged “onomatopoeia” would be steady throughout – they they are among the most-used tools in Tolkien’s kit.  Here they are, fairly steady, with two delicious peaks:

There’s Gollum!  His delicious sounds which follow that star in Chapter 5 and the goblins’ “Clap! Snap!” and battle sounds before it are the peak of the sound-imagery of the work.  Both here and at the local peak in Chapter 9 (which includes snoring, bumping, and grumbling), the uncommon sound words are so plentiful as to account for a good deal of those particular peaks in the uncommon word (red) line.