I've got a problem I've never encountered before and don't know how to solve it. The A string on my Fender MIM Fretless Jazz rattles whenever I play it open (but not any other time). Dampening the string between the tunner and the nut with my finger stops it, but I only mention that as a point of reference. It seems like the rattling is coming from the tuning machine, but none of them feel loose to me. I should also mention that the bass is strung with medium guage roundwounds. I was thinking it might have something to do with the nut or the neck relief, and/or to do with the fact that the bass is set up for flatwound strings (higher tension). Honestly, though, I don't have a clue what is causing this or how to resolve it. Please, any help or observations would be greatly appreciated.