It turns out that “chat” is the “onomatopoeic abbreviation of chatter”, and “chatter” is itself onomatopoeic.  I find this connection between Gollum and the Master of Laketown eerily satisfying.

• 05.022 and chats with it a bitsy,
• 14.030 but the Master ground his chattering teeth.

“chat, v.1.” OED Online. Oxford University Press, March 2015. Web. 22 May 2015.

We only hear buzzing on the approach to Beorn’s home.

• 07.025 There was a buzzing

Another word which sounds just like what it means.  Almost half of these bumps are from Chapter 9.

• 03.012 or bumped his nose on the pony’s neck.
• 04.045 said poor Mr. Baggins bumping up
• 04.051 bumped his head on hard rock,
• 05.008 not when their heads have recovered from being bumped.
• 05.144 and avoid being bumped into,
• 05.145 who could not make out what he had bumped into,
• 07.108 and had rolled down with a bump from the platform on to the floor.
• 08.059 bumping crash into trees,
• 09.011 or for fear of being bumped into
• 09.029 one of them bumped into another,
• 09.038 and the bumping they would get inside,
• 09.049 Down they go, down they bump!
• 09.050 and a bump,
• 09.056 and bumping mass of casks
• 09.061 though a few went on to bump against the stony pier.
• 13.015 when Bilbo fell forward down the step with a bump into the hall,

Bubble is another imitative word, and it’s used to describe the water in a barrel-floating scene and a mysterious sound when Bilbo is approaching Smaug.  Now the barrel scenes are funny and Smaug is pretty high and dangerous but in this particular paragraph, Smaug’s rumblings are likened to the purring of a giant tomcat.  Definitely part of a funny, tension-breaking image, and we’ll tag this word as “low”.

• 10.001 and bubbling.
• 12.011 a sort of bubbling

“bubble, v.” OED Online. Oxford University Press, March 2015. Web. 22 May 2015.

This tinkle comes when Bilbo stumbles over a golden thing, therefore I give it a “low” tag, but the tinkle of broken glass upon the entry of a foe could have a very different register.

• 13.018 and a tinkle

“Thump” is another word whose register depends on what is causing the thumping.  Dwarves threaten to thump plates in Chapter 1 and trolls thump about in a slapstick fight scene in Chapter 2.  While the mood is eerie in Chapter 8, the thumping of feet or of dwarves to the ground are clumsy and low in comparison to the menacing, listening place.  In Chapter 9, Bilbo’s heart thumps during the daring rescue – but their danger is  not particularly great.

• 01.067 Pound them up with a thumping pole;
• 02.070 and thumping,
• 08.001 and the quiet was so deep that their feet seemed to thump along
• 08.105 thump to the ground
• 09.029 Bilbo’s heart thumped every time

“Thud” seems to be an onomatopoeic word the register of which depends on exactly what is doing the thudding.  The feet of oliphaunts on their way to war?  Dangerous and high.  Barrels full of dwarves?  Low.

• 09.050 thudding on top of ones below,

Bilbo’s stammer in Chapter 18 points out the distance between his station and Thranduil’s.

• 18.046 said Bilbo stammering

A squeal is another sound made by prey animals and vulnerable persona when in danger.  Definitely “low”.

• 05.056 Give me time!’ But all that came out with a sudden squeal was: