I don`t know what old carpet smell is, but is is something like old linens sitting in a closet ;-)
Not that I would have any stinky interior auto carpet on my radar. I mean even the street rain, etc that we get on the most used seats - front ds and passgner floormats don`t stink
Is it the mats, or the carpet underneath ?
Like Autopian`s and how we interpret paint, it may take a keen eye to know if you have a leak thats inside. Once inside....that foam is dense and soaks up of water - that doesn`t really dry since it`s dense. Just FWIW. Which may not be in ones radar if you don`t lift up your carpet mats after the car is outside after a heavy rainy day, etc. Just FWIW