My two cents worth, john, which I know sounds nuts, but........
Something fell off, something which caused the unbalance; what ever it was, was in the rotating bits of the motor. Maybe, just maybe, there was a lump of mud or similar stuck in the fan, which just maybe could have glued itself in there somewhere, until it finally and completely dried out and fell off. You identified it at about 2000 rpm which sounds very harmonic, so by illogical reasoning maybe the unbalancer was in the harmonic balancer -howzat for reasoning?
Don't worry about - unless you overheat, then it was the fan, and its gone into orbit.
Strange things happen...
Cheers, GQ


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