When I first came to grad school, I was assigned to sit in on a class with some computer labs and to assist students with the labs. The course was a standard calculus course, except for the addition of some numerical/graphical work in the labs. The teacher I was assigned to was a nice person, but I came to realize he was not the most exciting. His class was not tremendously interactive, and he was pretty laid-back rather than particularly personable or energetic. (My own undergraduate teachers had mostly been awfully good, so this was a little different for me.)
After a while, I came to notice that when you sat in his class long enough, the sucking (or maybe blowing) of the air vents gradually became overwhelming. There's an ongoing drone about the limit of this and the derivative of that, and an incessant hiss low in the background. It's a bit like the never-ending whoosh in the background of Star Trek episodes, representing the hum of the engines. But when there's nothing much else interesting going on, the hiss becomes overwhelming.
Now years later I observe grad students who are teaching, and offer them observations. And sometimes the main observation I make while watching someone else's class is "Man, those air vents are really sucking today." Then I have the challenge of translating this into a constructive comment.