I have a 2001 tahoe w/74000miles. After I shut down the engine, a pump or fan noise from inside the engine continues to hum or turn. The only way to get it to stop is to disconnect the battery. After letting it sit for a few hours, I reconnected the battery and the noise started up again. Is it a fuel,oil, or water pump??? This is the first time this has ever happened. I know it's not the radiator fan.