I'd check your rear hub bearings, if your bike is inexpensive and the wheel bearings are not sealed, a ball bearing may have escaped the race and is rattling around. These seldom have enough grease when new. No biggie though, just repack your hub. The shop you bought it from should do this free. If youre on your own, you need to invest in some cone wrenches to do it. Another possibility, if the noise seems to come from your rim and not the hub is a loose spoke nipple rattling around inside the rim. Maybe one got dropped inside during the wheel build. Most likely a loose bearing in the hub though.