Knocking noises are generally not good. The good news in your case is that your Malibu may still be covered by the factory warranty. Make sure your engine oil is full - check the level using the oil dipstick. You may wish to take your car to your local GM dealer and have them find out what is causing the noise and if your warranty will cover the repair. Please post the fault with your care after the problem is diagnosed and if GM will cover the repair or not.