Is the noise only present when the ignition is turned on? If so it is more than likely the fuel pump you're hearing.
Another thing you can try is leaving the ignition on for a few minutes (engine off) to let the fuel get up to pressure - If the noise then stops, it's the pump you are indeed hearing.
Noise starts when ignition is turned on and continues all the time
It's the fuel pump then.After the ignition has been turned on for about a minute the noise stops, but one the engine is started the noise starts again and does not stop while engine is running