K 9, I don't mean to steal Bearfoil or Turbine Ds 'thunder' but here is my take on all that clattering going on from that big 'wheel' in the wind.
During the fan's rotation in the wind, the top blade at the 12 o'clock position
going downhill, falls over towards it's neighboring blade due to the increasing angular separation 'veeing' towards the blade tip. Each blade has what's called a mid-span snubber which is a proturberance (sp?) chordwise across the blade about an inch deep on some engines. This is the first contact point on each blade against which each snubber which makes the clattering noise. When the engine is running the centrifugal force separates each blade equidistant from it's neighbor with a clearance.