I have an 07 2.3 AT and I notice that when I come to a stop (with the engine still running) I hear this metallic pinging sound. It sounds sort of like someone dropping BBs into a tin can, the sounds come at an interval of about 3 seconds, and the sounds seem to come from behind me. After I turn off the car and get out, I can hear similar sounds from outside the car that seem to come from near the exhaust. Any ideas what's going on or if this is serious?