K_9
unmanned transport gave a good explanation of the fan blade clanking heard on a windmilling fan. I think, but not positively sure, this was confined to the early CF6-6 engines on DC-10 aircraft. The fan blades are intentionally not held tightly in the fan disk slots when the fan is at rest (not rotating). Upon spool-up, the centrifugal force moves the blades outward, firmly and evenly seating the fan blade upper dovetail surface to the corresponding disk lobe surface.
Now a little history: When the DC-10 was introduced to revenue service, some airports didn't have either enough jetways or ones they had couldn't be used to board passengers on DC-10s. Also, many terminals were undergoing changes/upgrades. So airlines used mobile stairs for boarding. The stairs were just in front of the engine and if it was windmilling, the clanking noise unnerved boarding passengers. So the airlines asked GE if they could stop the clanking. GE redesigned the fan blade retainer component to stop the clanking but still permit proper seating of the blade upon spool-up. This modification was introduced throughout the DC-10 fleet. This anti-clank modification feature became standard in the CF6-50 & CF6-80A/C engines.