knock noise near gas filler

I am new to this site. I think this may have been asked before. I have a 2007 e350 4matic non sport and it makes a knocking noise when I am stopped at a red light. Knock...a few seconds...knock etc, ect. It seems to be coming from the area near the gas cap. I have brought it in to the dealer about 3 times for this and they say that they don't hear anything. I know that it doesn't happen every day but it does happen more often than not. Apparently there are no codes or problems. I know it is a real issue and hope that the dealer isn't just pushing me aside as a crazy person. I know I have read other issues about this. So either we are all crazy or the dealer knows there is an issue and is just blowing me off. Can anyone explain to me what this is and exactly how to fix it. Although I generally get this car serviced at the dealer I am mechanically inclined and could probably fix this on my own. It is just annoying and an embarrassment on an otherwise beautiful car.