my fuel tank howls on really hot days with some brands of premium fuel. i use only premium but if i use cheaper gas i hear the noise less. i have a non vented cap which is spec for my car. im wondering what is the little plastic mushroom shaped plastic piece above the fuel tank near the lines. it looks like some sort of vent or valve. what is it, what does it do & does it go bad or make noise? im hoping its not my pump as i just had that replaced about a year ago. the noise starts and stops when the key is turned on/off but is totally intermittant and some days just doesnt happen at all.