I assume that there are not any broken spokes in the rear wheel? Also be sure that the derailleur hanger is straight so that it does not ping the spokes as the wheel goes round. If all is well there then try putting a drop of oil at each nipple where it enters the rim and where the spokes cross each other, sometimes as the wheel is loaded in certain ways the spokes rub on each other and make noise. If you have a reflector on the wheel take it off - sometimes they cause noise. Spoke protectors (the frisbee shaped thingy behind the freewheel or cassette) can also make noise if they are installed improperly or are broken so check that too. Good luck.

-Ryan