I was in my office parking lot today when I heard a TING-TING-TING. I looked around and saw a truck with a tall exhaust pipe at the corner of the cab, as trucks are wont to have. On the top of the exhaust pipe was a metal cover that kept flapping up and down, as the exhaust would blow it up and then the dispersion of pressure would allow it to drop down again.
I’ve been seeing these as long as I can remember on trucks and construction equipment. Do they serve any practical purpose at all? I can’t imagine what it might be, other than to give a quick visual confirmation that the engine is running (although how you could not know that a diesel engine is running is beyond me).