Wow... I bet that's annoying.... I would guess this isn't normal. Is it on all the keys??? Maybe some packing material got down in between a few. I played the S-80 at Guitar Center for quite some time and never noticed this.... then again if it's fairly quiet I wouldn't have heard it over everything else going on in there. Maybe you could get the store that sold it to you to give you their opinion. I'm no expert here... but this situation doesn't sound good.

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John 3:16 For God so loved the world, that He gave His only Son, that whoever believes in Him should not perish, but have everlasting life.