wow, you don't hear about that very often but i have read where the plug has worked it's way out for sure. i would guess, like you mentioned, that the plug from the compression was shot out hitting causing the sound you heard. just make sure that it is tight and enjoy your running bike lol.
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08' sportsman 800
07' yamaha vstar 1300
07'yamaha vector
06'yamaha apex
08' gmc z71
09' subaru legacy R
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