Q: Car won't stay on for less than 5 seconds and makes a knocking sound.

Hi there. The symptoms that you have presented sounds as if the fuel pump has stopped working. When the fuel pump stops working, the engine will start up and run rough and stall out. While the engine is running, due to the lack of fuel entering the engine, the engine will make a popping sound and a knocking sound which is the engine misfiring and trying to operate on very little fuel with a mass volume of air. Check the fuel pump relay and see if its clicking on as the key is turned on. If the relay is not clicking on, then check to see if there is power to the relay. If there is no power to the relay, then there is a break between the relay and the ignition switch. If there is power to the relay but nothing past the relay, then the relay needs replaced. If the relay is clicking but the fuel pump does not work, then check the fuel pump fuse. If the fuse is good, then either there is a break in the wiring between the relay and the fuel pump or the fuel pump has failed and would need replaced. If you need further assistance with your engine stalling, then seek out a professional, such as one from Your Mechanic, to help you.