hello, I have a 2012 Cruze LT and recently noticed a thumping sound if I am waiting for someone and have car in idle and after turning the car off. It happens every 20 seconds or so but eventually goes away. I took it into a chevy dealership for service and asked tech about it and they said they were unable to duplicate the noise yet I hear it every time I turn off the engine. Has anyone had this issue before so I can tell dealer what to fix? Thank you.