If you go back to your OP on this I mentioned this as one of the possible causes of your rattling.

http://www.xt660.com/showthread.php?t=23700

As I said then, it's the decompressor pin that is usually at fault - either loose, worn, broken etc. It is a known fault but not all that common. You will need to investigate the mechanism and replace the pin.

You can inspect and check the operation of centrifugal weights and spring easily enough on the cam sprocket, but you will need to access the camshaft itself to inspect the decompression pin (3) and the shaft (4) which links the centrifugal weights to the pin (and the correct operation of the whole decompression system).

Here's a picture that may help...

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