It could be a leaking exhaust system, or your fuel lines leaking. I would get a flashlight and look under the car for any obvious signs of leaking gas from the gas lines. For the exhaust, you can make soapy water(1 part dish soap to 30 parts water) and spray it on the exhaust pipe, especially near the exhaust manifold and other connections. Do this while the engine is running. Repairs are best done at a good auto shop.