2006 Chevy Cobalt Front end knocking when going over bumps or rough roads

I have a 2006 Chevy Cobalt LT Coupe and my front left side of car makes knocking sound as if something is lose when I go over rough roads, bumps, and potholes. What is causing this? Also, my car putters when in park and goes through a quarter tank of gas traveling at less than 20 miles and I just performed a tune up with spark plugs and wires. What could be causing this? I have over 83,000 miles on this car and bought it used.