I have an 05 2.2 Ecotec Cobalt giving me some trouble. It is my girlfriends car and she noticed some weird knocking noise about a week ago when the AC was turned on. A day later the noise was knocking without the AC on. It is coming from the passenger side of the motor and I have replaced the timing tensioner and took the belt off of the AC to see if that was it and I still have to same noise. When we had the valve cover off everything seemed to be in working order and no damage or wear. The oil was a little over a quart low when I first started looking at it and I did change to oil to see if anything was out of the ordinary but everything fine and nothing unusual about the oil. The knock is NOT a consistent tap tap tap tap. Whenever I start the car it usually does not start making noise until it idles down the whole way. When ever i rev it up it either gets quieter or goes away completely. When the AC is on at idle it gets worse due to the extra drag on the motor. I am fairly stumped on this one and the next idea I have is possibly the water pump. I am hoping to either fix it or quiet it up enough to trade in because last summer I believe I replaced every damn suspension component on this car. Anyways... anyone have any insight?