Tank Filler Pipe And Cap problems of the 1997 BMW 740

Hi, I am not sure if this is a safety defect or not. I had a sound like an expanding and contracting of metal in the back underneath of car. It sounded like a cookie sheet when it is first heating up in the oven and it expands and contracts. The dealer couldn't find anything when I brought it in but said that they were going to call BMW technical support because they had a theory that a spider was clogging up the fuel line and that this had happened once before to their knowledge and resulted in the gas tank collapsing. I brought car in again and they said they removed a spider and put in a new pipe/line. The sound I had heard before was gone. Some people have told me, however, that the whole system is a closed system and that this couldn't have happened. Is it possible? is problem fixed?.