We had a similar sound but it only occurred when you were hitting the brakes and decelerating. They had the car for one month, gave us a nice loaner. They had to bring in the factory technicians at great cost to GM. Turns out there wads a baffle that was not properly installed at the factory and other pieces were hiding it from the dealer. They had to take the wheels off in the front and touch everything until they found the bugger. It had been installed backwards and so was not attached to where it was supposed to be. Jesus.