Thanks Catchlights.some dust inside the lens is inevitable, and most of the time it won't affect the image quality unless it is very very bad.
if you send it for clearing, it will come back again, it wouldn't be 100% clean.
learn to live with it.
if you insist, can send it to the Nikon Service Center or contact fatigue for a quote
I like to quote this..Thanks Catchlights.
I agree on what he said: learn to live with it and enjoy shooting
It's not that expensive to have it removed but if you send it for cleaning every time there's dust, it would cost you a lot.