I too would also suggest checking your valve clearances and, while you're at it, check the operation of the decompression mechanism and particularly the pin in the camshaft. The pin is fairly well known for coming loose, dislodged, misaligned and causing mysterious top end rattles. The decompression mechanism can be accessed assessed through the camshaft sprocket inspection cover on the LHS of the cylinder head (which you will have to remove anyway to do the valve adjustment.)

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