so my cobalt has always had a slight valve tap when i first start it but when it warns up it gies away. now i have this knockin sound comin from my motor it doesnt do it all the time just randomly it will do it its hard to explain the sound its just like i clankin metal or somethin. wonderin if anyone knows what this is or has this problem. no loss of power or anythign like that just the noise. any suggestions

 

My guess without hearing it would be the timing chain ,replacing the tensioner is what probably needs to be done.