Chevy86 IROC-Z

iTrader: (17)

Sorry guys but there is no board for gas tanks but I got a serious question. Close to a year ago, I bought a gas tank from the junk yard. When I was cleaning out the tank with clean gas, I heard something moving around as I shook the tank. To make a long story short, I pulled out a round piece of plastic about 3 inches across, and a 1 inch hole right smack in the middle. The color was a clear tan. What is this piece?
I think it has something to do with my fuel pump whine when at half tank. The pictures are here in this thread.
https://www.thirdgen.org/forums/tpi/...-question.html