Tell you what - there's a lot more to this story - the strings I'm working with right now are Larsen Virtuoso - sooooo - hmmm - they are lower tension than any other string I have ever used...   right ?  

Well - when playing FF etc on strings, and adjacent fingernail on the next-string-up - and "in place for the next cross-over note" would get hit by the wider vibrations on the lower string being played, and "buzz" a bit....   That never happened before....

Paying a bit more attention to finger placement and "shape" cured it in a day or so. Cool - all new to me, but not unexpected in any way.  I understand that.  Simple physics.   And I do LOVE the dynamic range of the Larsens, don't get me wrong - they have the best PPP response compared to any other string I've tried so far...  it just "grips" with the lightest touch and sings...  (very softly - I like it!)

But the damnedest thing is, the E would whistle at me - not regularly - but on occasion - and "generally" ( hmm, but not ALWAYS ) on a down-bow slur from a fingered (or open) A string note.   I COULD accommodate it by very specifically "digging harder" - but - nahhhh - not for me.

I should also say - it is NOT and is NOTHING to do with lower part of index finger touching the E accidentally - I've run hours ( ok, I exaggerate - but several tens-of-minutes ) just gliding from open A to open E and - like 1 in 10 times or so, the little sod will whistle at me....   grrrr ....

I just KNOW it must be me really, but darned if I know why...    Tried both a "reasonably well used" ( hmmm... 500-700 hours use maybe ) bow, and a "new" (relatively) with definitely little more than 50 hours use - I keep it separate, for the very purpose of just trying to establish if or when I need either a re-hair or just a new bow...).   Different rosins as well - same thing happens....

And, I've already used the same Kaplan E string on other sets - so the Kaplan E is not new to me by any means, and it's not just the Larsens this happens on with me - I'm just puzzled about the whole thing....  other folks simply say they don't ever have an issue, regardless of string type...   weird or what ????

Anyway - I know that whatever is wrong with my bowing, the Kaplan E sorts it out... LOLOL, and for now, I'll live with that !!!!!!

Cool !