* Fuel Pump: This is a very likely culprit. The fuel pump is located near the gas tank and activates when the ignition is turned on (to prime the system) and runs while the engine is running. A failing fuel pump can make various noises, including humming, whining, or buzzing. A loud clicking might indicate a failing relay or a problem with the pump's electrical connection.
* Fuel Pump Relay: The relay that controls the fuel pump could be failing. A clicking sound is a common symptom of a bad relay.
* Exhaust Leak: A leak near the rear of the exhaust system could produce a hissing or rumbling sound, especially noticeable when the engine is running.
* Suspension Components: While less likely to be directly related to the key turning, worn suspension bushings or other components near the rear could create noises that are more noticeable when the engine is running due to vibrations.
* Brake System: Although less likely, a component in the rear brake system, such as a sticking caliper or a worn brake pad, could create a noise that's amplified when the car is running.
* Other Electrical Components: There might be other electrical components near the gas tank that are malfunctioning. This is less likely, but worth considering if the sound is electrical in nature.
What to do:
1. Describe the sound: Is it a clicking, whirring, buzzing, grinding, hissing, or something else? The more detail you can provide, the better.
2. Check the fuel: Make sure your gas tank isn't nearly empty, as a low fuel level sometimes amplifies sounds from the fuel pump.
3. Listen carefully: Try to pinpoint the exact location of the sound. Is it directly from the tank, or slightly offset?
4. Professional inspection: Given the potential for a fuel pump issue (which is safety-critical), it's best to have a qualified mechanic diagnose the problem. They have the tools and expertise to safely identify and fix the source of the sound. Ignoring it could lead to a more serious and costly repair later.
Don't attempt any repairs yourself unless you have significant automotive experience. Working around the fuel system is dangerous if not done correctly.
