OIL?
Did you buy your bike new? Did it get an oil change already? Did it get Quality oil and Filter?

Won't cost much to change it again anyhow, just to see if it changes the sound any. Could be the oil has started to break down or the filter isn't doing it's thing.

If that fails to quiet it down, get some LOUD PIPES! :) Just kidding, with fresh oil and filter I would then ask the local mechanic if what you hear is "normal".