Most likely explanation is that part of the tinware unser the deck has become detatched due to either foreign object damage, or corrosion, and is coming nto contact with the blade. Easy to check.
If that's not it, it's possible for something to have found it's way inside the coolng shroud of the engine. You will need to remove it to check.
phil_saunders(a)bigfoot.com
Phil Saunders
November 2009
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