what is it my 97 trooper knocking sound from back of motor

If your service light is flashing then it means you are under a miss fire condition that can damage your catalytic convertors. The knocking sound is the unburnt fuel from the miss firing cylinders, exploding in you catalytic convertors. This is serious because if the cats are ruined it will not be cheap. I would recommend driving it to a repair shop and get a diagnostic to see why it is miss firing. Most likely it needs a tune up. Then they can check the cats for damage when the miss is fixed. Good Luck!