I see lots of "carbon knock" issues during the cold weather months. GM Top Engine Cleaner works great. Just pull a spark plug out of each cylinder and pour it directly into the cylinder. Let it sit over night and then crank the engine to empty the cylinders. Re-install the plugs and take it for a good ride. Don't forget to change the oil and filter when you're done. Between September and April I do several cars a week using this method and it works great. If you don't want to use the GM product, Techron fuel additive is a good second choice. It'll get the job done, but it takes awhile.