TOYOTA PRIUS KNOCKING ON START UP???

Customer concern of a terrible knocking noise when started. I was unable to duplicate this. The customer sent me a KZbin video (sorry I didn't save the link). The customer claims her car is doing the exact same thing... the cause in that video is a head gasket leaking into the cylinder and hydro locking when the engine is started. With multiple tests, I was unsuccessful in proving a leaking head gasket. If anyone has any tips or ideas hook me up!!!