All newly built aircraft will have a certain amount of debris left behind at construction even after careful cleaning.
When I was being trained back in the 1960's we were told of a case where a loud clonking noise was heard while the integral fuel tanks in a wing were being 'shoshed' with liquid coating.
On investigation a vacuum cleaner was found, having been overlooked when the final skin was riveted in place.
FOD doesn't get much bigger than that!