A sound of onomatopoeic origin, and indicating such tiredness that we’re labeling it “low”.

• 07.024 and down dale they plodded.
• 11.004 died away to a plodding gloom.
• 13.065 and they plodded slowly

“plod, v.1.” OED Online. Oxford University Press, March 2015. Web. 27 May 2015.