when I can record the sound since it doesn't alsways happen and post it. But at idle I can hear a very loud almost matal on metal knocking coming from engine bay. Now last night O poped the hood and nothing seemed loose but enegine and a tapping noise from inside it...any ideas?? And I know this is very very vague .

 

could be the the timing chain tensioner. Usally the first thing to go on all cobalts, i dont think i've met someone with a cobalt and didnt have that issue. Bring it to the dealership. People correct me if im wrong.