I just noticed this after my last two gigs taking the bass out of my car. Just on the olive G and D strings, my other 2 none gut strings are fine. I have had them on for about 5 months and a little more than 50 shows. It seems to me that it is almost like the core is pushing through the wrap, but after i wipe it off i can't see any gaps in the winding and it has not come loose at all. It feels like fairly fine sand spiraling up and down the winding of the strings. Its not much and it all wipes off easily and the string sounds fine and it all looks to be ok, for now. I'd just like to know if any one knows what i am talking about and how bad it is?