With a headunit that has a sub pre-out, when it senses the amp and sub being there, it'll change the amount of bass being sent to the door speakers. At least that's how i figure it. lol. I had the same problem with rattling crank handles....then in went the sub and hearing the rattle is a thing of the past.
New cranks are only like $15 from the dealership brand spankin' new. When that 'roller' is loose from use over time, it's what rattles. Going new, they are tight and have very little play.