Make sure the repair shop has all the information that you have. You may not know what's wrong with it, but you do know when something is wrong with it. If the sound persists, you need to get the car inspected. Isolate the sound. If you have a leak, you might want to touch your finger to the spot on the ground and sniff the fluid.
Most people can recognize the smell of gasoline.
Coolant smells sweet. The smell of burning rubber may indicate overheated brakes or clutch. A sick catalytic converter often announces itself with the smell of rotten eggs. If so, you may have a problem with your battery, alternator, brakes, engine temperature, or oil pressure. Watch, too, for other signs of a problem, such as a leak.