When car is warmed up, and either reversing or driving out of 1st gear there is a loud ping sound coming from under the car. Sometimes it will ping twice. Sounds like front and sometimes back. I dropped the driveline and took to mechanic and he said the ujoints were good. 1 of the 3 though had a tiny bit of play but not enough to call bad. Trans, and differential have new fluid, when changed there were no metal shavings. The pings sound like if you hit a frying pan with a small hammer. What could the problem be???