mtmd is MobileTimeMachineDaemon, and it's either making a Local snapshot or monitoring free space and deleting backups if there's not enough room.
See #30 in Time Machine - Frequently Asked Questions.
I followed the suggestion below, and so far it has worked. Will post again if the problem returns.
I have a 2012 Mac Air, OSX 10.8.2May 13, 2012 1:20 AM
I have a MBP 15" (June 2010) that runs OS X Lion. Under v10.7.3 I had problems with mtmd and mds doing huge CPU loads.
This was caused by the Spotligth index. I had to delete the indices (by marking them private) and rebuild them (remove them from the private list). Booted in-between. I deleted both the "Macintosh HD" and "Time Capsule" indices.
I am currently working with 100.000 files at the same time. Transfering xml's to html and then moving the directory, except I copied it instead of moved it (mistake). All by my java program and not manually.
300.000 files made the mtmd process go bananas and my fans made my macbook hover around on the kitchen table ;-)
Setting the directories containing all those files to private in spotlight made the mac land safely in the table againg.