He's going to whine about the noise his drill makes.
Panel 1: We see him drilling into the jawline of another merry Milborough mutant. Oddly enough, the drill's sound is represented by "WHeeeeeEEEEEeeeeEEEEEEEE" instead of "Drill! Grind! Scrape! Scronk!"
Panel 2: As he continues to drill, the assistant prepares another instrument.
Panel 3: It's now the end of his work day; he reminds of this by saying it's been a long day. His assistant, who looks like the April of 2008 with Dee's hair color, asks him if something is wrong. He says there is.
Panel 4: He's not hearing the high-pitched grinding sound any more. That's right; he's not only used to the drill's sound, he's addicted to it. Of course, since he has his fingers in his ears as he says the punchline, I could be right about the gas tank leaking.
Summary: Lynn has turned John into the dentist from Little Shop of Horrors.