Why does my 09 chevy cobalt make a knocking noise under the hood when I start mycar

Hello, that doesn't sound great. I would strongly recommend not driving the vehicle further and having a local shop and technician near you assess your vehicle to find out what the noise might be. If it's internal to the engine, it could be a big problem. Most technicians who know what they're doing can figure out a general sense of what the noise is and give you advice. Use this link to find some shops near you that can help with your concern: www.repairpal.com/repair-shops