Engine knocks after turning car off?

Engine is probably running backwards,normally due to engine idle is too high. If vehicle is carburated readjust idle to manufactures specification,normally located on info tag on radiator support under hood. If vehicle is fuel injected/computer controlled check idle air controller for erratic operation,this part gets carbon build-up,especially in high mileage situations.Part is located close to throttle body(where throttle cables hook to) remove and check for carbon,if a large amount of carbon is found replace controller,part usally totally fails after being cleaned.Hope this info is helpfull.