While slowing down as I approach traffic signals.

While slowing down as I approach traffic signals or sometimes when taking a corner, there is a noise that sounds like a clunk or kick-back. Sometimes loud sometimes softer. It can be replicated easily by coming to an abrupt stop but cannot be replicated when the car is put into third gear of an automatic transmission. Other than that, the car drives wonderfully, has new brakes and is serviced regularly for fluid changes.
We are having difficulty getting an accurate diagnosis and we've already spent over $300.
Please help!