by a passing plane,和by a knock at the door

介词 by 后面的宾语各种各样,加 a/an 表达“一个”毫无问题啊,比如：
The following program will be presented by a group of well known scientists.一群著名科学家
This film is directed by an American.一个美国人那就是by加交通工具时不加a了？by 后面加不加 a 要视情况由语境来定的，无法一概而论。你指的是交通方式， 不是工具；有些句子的意思即便是by 后面是 car，train 这些也仍然要加的，关键在于前面的动词是什么。比如：His sister was hit by a car yesterday morning.他姐姐昨天早上被一部车给撞了。We will be going to Shanghai by train.这种自然不加，指的是用什么方式...