Recently I have noticed a loud noise that usually starts and last say 15 seconds, then dies. I always assumed it was the radiator fan running. As the noise sometimes started as soon as the car started, I began to suspect something was up. It's winter and the engine is darn cold. So this morning I raised the hood before starting the car. It did not start making the noise right away but did after the car began to warm up. It doesn't appear to be the coolant fan. The fan was not turning and the noise seemed to be coming from the area under the battery. Is there something down there and is it something I need to be concerned about? When it is running, it sounds like a jet engine and seems to shut down the same way. Not quickly but like an electric motor drifting to a stop. Thanks for any help or ideas.