Thais make a lot of odd noises while hitting bags and pads (and trees).
Yod: http://www.youtube.com/watch?v=l3h-JvHCpWM
Kem: http://www.youtube.com/watch?v=HXlk_szOx34
A younger fighter from Kaetkhamtorn: http://www.youtube.com/watch?v=nIYeJ8_Mqdw
It's a banana "tree." They're actually not woody, as sweepings rightly notes. Rather, the trunk has a growth habit not unlike that of an onion, essentially a bundle of leaves wrapped around themselves.
PROTIP: When cutting a path through a thicket of Cuban Red bananas to get to the one you want to fell, do not attempt to slash through the trunk of a dead and rotting banana plant trunk, which is mostly air when the plant is alive. The accumulated rainfall that has drained into the trunk and putrefied to a high stench in the hot climate comes gushing out of the cut under the pressure of gravity in a remarkably effective spray pattern. Comical for others, but unpleasant for you. The more you know.
It's worth noting that the Japanese got so tired of Buakaw beating the shit out of their fighters in K-1 that they modified the rules in order to cripple one of muay Thai's greatest assets, which is the clinch.
The Japanese simply ran into a challenge and, like other great athletes throughout history, adapted their strategy to defeat that obstacle...with Microsoft Word and lawyers.