Engine knocking sound

My car started making a knocking sound. I checked the oil nothing on the stick so I added oil noise stopped. Then 2 days later started again, checked oil still full. There is no engine light on, there is no smoking, there is no loss of power and the knocking starts when you crank the car. While you are driving it will go in and out and when you come to a stop it will stop, but as soon as you push on the gas it starts knocking again. The engine has not shut off at all it cranks every time I start it. The oil light never came on it has not overheated I have had to change my brake pads 2 times in 6 months. I have had the car for 3 years and I have never changes spark plugs or wires and I have only done 2 oil changes.