Could be many things
Stone stuck between a a brake disc and back plate
Wheel bearing
Drive shaft coupling
With care watch rear mirror and with no one behind very gently apply hand brake does the noise go away(Do this in a empty car park) If it does then problem may be from rear brakes.
Also while in car park when the noise appears turn steering wheel slightly to left or right if noise goes away could be a wheel bearing (fault bearing is normally on the opposite side to which you turn away. ie if you turn left and the noise goes away then it may be the right wheel bearing.
See how it goes
Maurice
April 2009