Flight Line noise sources easy to confuse
While an operating engine produces quite a loud noise signature, on a busy flight line it is easy to confuse which aircraft/engines are generating the engine noise that is being heard. Ear protection further impacts the ability to determine a noise source.
One thing I have found curious (my nerdliness is showing here) is to notice that turbofans on parked jets often windmill in the wrong direction. Planes at passenger gates (head in toward a several story building) often see a localized tail wind that causes the fans to turn backwards.