usually when you hear it. it means that your sound deading material is waring out or the fuel is really low than what the gas guage is reading. other than that i would check out every thing as in check the bands of the tand as well as all of the tubes. i had to replace my filler neck on my mustang because i was hearing the same thing. i would check that as well. to see if there is no rust marks or any thing out of place or out of the normal collor.