I realise that the fluff ball solved it but just as a curiosity did you try changing the brace height? Since it's a straight bow and the string can't vibrate against anything it pretty much has to be harmonics. I know nothing about that except that the string tension and length contribute to the noise. Maybe a guitar player can help. You really can't change the string tension that much(I don't think) but BH will change the length a tiny bit. Would it be enough or would it just raise or lower the pitch? I'm not a big fan of fluff balls although they work great. I have used a 2" piece of brain tan (thanks BowEd, where is he?) lace just wrapped in the string and it works and is less obtrusive but I would rather have nothing.
