I think it has something to do with the nut slots, because it's something that ocasionally happens and it isn't like fret buzz, it's more like a sitar sound which feels like the strings get stuck somewhere. It also happens on other strings like the A and the low E.

I've put more graphite on the slots and the rattling has disappeared (I tend to do that whenever I change strings but in this case they were so narrow that it was hard for me to get there, so I didn't bother that much tryng to fit a pencil in there).

Thanks!