Why does your Harley sportster ping?

If it is after the engine has turned of it's due to metal contraction. Metals expand when they're warm and when they cool down they return to their original size. This shrinking and expanding can make some parts(particulary exhausts) to move with refence to other parts. If the parts aren't able to move freely they'll take out their movements in short jerks, which results in the pings. ---- If it happens while driving, try gas with higher octane.