wierd engine sound..piston slap?

I have an '03 yz450. When i let the engine idle there is a slight knocking or slapping sound from my engine. Once i crack the throttle the noise seems to go away, but that might just because the exhaust is too loud to hear it.

Anyone have any idea's?

If you have an alum. skid plate alot of noise is radiated upwards,making insignifigant noises seem scary.

Check your kickstarter lever (above part # 23: http://oem.thumpertalk.com/2003YZ450Fkickstarter.aspx ) to see if it's rattling from that location? Worth the check. Mine was making alot of noise from there and I thought that I had some issues with my engine before I caught it. If that's it, grease that fitting.

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