Loud ping when selecting reverse

[Yoshi Yaris]

I have a Feb 2010 1.3 5dr Yaris YR Auto with 1,500 on the clock. At 1,000km service I advised dealer of a loud, intermittent ping when placing into reverse (it sounds like it's coming from somewhere in line with the wheels), particularly when cold and following being parked overnight (it's parked on flat and almost sounds like some sort of metal pin disconnecting). They adjusted the brakes, however, still have the problem and seems to be happening more frequently. Have contacted the dealer and going back to see if they can determine the problem.

Any advice. They don't seem to be aware of any existing problem.