i dont think that it would be missing any blades, as you would notice the vibration at any rpm level, a out of balance fan is a very violent vibration.

fan having play in it, would be more likely as rpm increases it would get worse as he fan speed picked up.

pop the hood with the engine in neutral, and pull your throttle to see if you can tell were it is coming from.

2000 SPORTY, DEF BOY customs, mmm....lockers, mmm... gears, mmm... double transfer case, mmm... stroker
http://www.cardomain.com/ride/2377991