This is probably a goofy suggestion . . . but have you tried shaking the culprit out through the vent holes? (I guess I'm thinking the rattle might've come after, rather than during manufacture. . . .)
Or you could pour something that'll solidify in the fork blade, though that's probably a goofy idea, too, since it may start to solidify before it reaches the rattle--and it's probably just a goofy idea, period.
spring green, wi