2008 GMC Envoy Knocking sound during initial startup

Abnormal (sluggish) cranking and knocking sound in the beginning of engine starting. After starting the engine runs normally in idle position. When it reaches normal running temperature and transmission is engaged to go , the knocking sound comes again as it takes off and dies down as the jeep starts to roll. During running if the throttle is pressed a little more we have the knocking sound again.