I'd stick with the catwhiskers like you've already got, just install them differently. If you just knot them on the string, the knot works loose and they fall apart in short order. Also, that leaves the legs too long and floppy, so they do make that 'whipping sound'.

Instead of tying them on like that, cut each of the sheets into 2" lengths. Then put one or two pieces on each side of the string, line the ends up and clip them in place. A small pair of hemostats is perfect for this job. Clip them out toward the ends! Tightly - very tightly - wrap serving thread or dental floss around the midpoint of your silencers, about 5-6 wraps is plenty. Just be sure you've cinched them down good, really compressing the rubber around the string. Then tie off and melt the ends of the thread to keep the knot from loosening. A drop of fletching glue could be added to the knot for insurance.

When you seperate the strands, instead of having long, nasty, flappy legs, the silencers will form nice little balls and won't make any noise of their own. They'll last as long as the string does. They also work just as well, if not better, than those expensive, fancy, solid rubber silencers. And those fancy ones won't last nearly as long.