Buzzing/whining sound from drivers rear behind fuel cap

Iv had this sound for a couple of months and it is a constant buzzing/whining sound coming from behind where my fuel cap is, I believe it could bet fuel pump from different symptoms iv read of other people vehicles. I start up the car n u here it straight away inside the car and if your close enough to the noisy area you can here it outside the car, I have noticed some days I get a lag in power in all gears but somedays it's fine, but the noise never stops it is constant, then goes off as soon as the engine is turned off. Any ideas?