This is the problem on these cars from the factory gm installed a liquid filled lower engine mount. When those burst there's a clunk when that happens. But most of the times you will not see fluid on the ground because its blown back and on to the car while driving. What you need to do is get a friend and have them hold the brake while they shift from park to reverse to drive if you see any weird movemenet or you feel that the movement doesnt look right. Take the car and get the lower engine mount checked. And there is one on the right side of the transmission that could also be bad as well
