Does the pitch of the noise change with speed change? Does it create any sparks at night? It's possibly a wheel bearing that is bad. They are inside the drum brakes and rotate around a spindle. So looking at the brakes does not help as you have to disassemble the bearing from the drum and check the race and spindle for scaring, and the bearings for flat spots after cheaning everything with grease remover. That is best done by a professional, as they know what to look for and have cleaning sinks to get the grease off.