The article discusses how the author experiences a rattle in her new car that greatly bothers her and her husband, but when she goes to get it fixed, the repairman cannot hear it and thinks she is crazy. She then relates this to everyone having their own "rattle" that others will invalidate and say that it doesn't exist, but deep down, bothers you to the root of your existence. She correlates her experience in a beautiful transition, "Dear Reader, you just might have a rattle, as well. Maybe not in your kinda, newish car. Rather, that thing that is the thorn in your side, that others can’t see or detect that you’re told to get over." So, the article isn't actually just about the noise in her car, but rather how she felt when she went to get it fixed and how she made that connect to her reader.
I think Holden's rattle is the loss of his brother. We even had to wait five chapters before Holden's character even breaks that news to us. He nonchalantly slides in, "He wrote [the poems] on it so that he'd have something to read when he was in the field and nobody was up at bat. He's dead now... You'd have liked him," (Salinger 43). Holden discusses a sentiment of Allie, his baseball glove, and then casually slides in the fact that he's dead from cancer, gives the date he died, and then continues talking about the glove, just like it was a minor detail. Holden presents this detail like a rattle. It's little, but we know its there by the loving description he gives of him, and that the loss has left him with a pain no one else knows.
We all have a rattle, maybe as big as Holden and the loss of his brother, or as small as the article's author and a little annoyance in her new car.