My car has been a good car, when I'm on freeway and going 80 and up hill, car sounds like pinging, or cheap gas noise. I really don't like adding liquids in my car. I had new plugs, oil change, new back shocks, serpintene belt, new front brake n rotors.
Something that comes to mind is carbon on the valves or combustion chamber. The oil consumption these cars engines suffer from can cause this and eventually the caked on stuff on the valves or inside the combustion chamber can cause pinging under load. Usually requires an induction cleaning. Not sure if you can do the cleaning using walnut shell blasting, is fairly common to have to do on European cars that use direct injection. I did an induction cleaning using a product from Seafoam specially made to spray through the throttle body. Worked well on my cars ive used it on. Although I had to clean the spark plugs afterwards.
Valve coking has never really been a problem with any of the Cobalts except the turbo Cobalt since the LNF is the only one that is direct injected. All the rest are port injected. A few people have done walnut blasting on the LNF, but no major improvements were noted.
OP have you changhed the fuel filter? maybe try running some seafoam through the fuel system and engine as well. I would try a different gas station. You could do compression tests and leak down and see if you have any issues going on in the engine.