I have a 1994 Z28 that I recently rebuilt the engine in. It has been running great without any issues. Then about two weeks ago the exhaust got knocked off the headers, that is what I get for lowering the front 2.8". So I took it to my exhaust shop and they just welded it together. Now it is sounding like I am getting some spark knock at low rpm's under a load. Any idea why this would happen? The only difference is having the exhaust actually working again. haha. There were no problems like this previously. Thank you in advance for your help.