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.