The apron can be a pretty loud place sometimes. I've worked at a cargo hub where there were often two or three turboprops with engines running at the same time so noise isn't necessarily an indication of which aircraft is 'live'.
Discontinuous black & white (flickering) concentric markings on the propeller blades seem to be the most effective warning of rotation.
Aircraft are like guns in some ways - perfectly safe until you forget that they're dangerous...