antman450,
The guys here have given you some good answers based on their experiences and you might like to examine all those possibilities.
The way you described the sound makes me think of another possibility. I've heard that sound while hovering also and it is a higher pitch than a rattle. The sound is coming from the prop and the reason you only hear it briefly is because the angle of the plane, the wind direction and engine RPM happen to be just right to make it audible at the location where your are standing. Its that same sound you hear coming from a helicopter rotor but it is a much higher frequency.