Thanks for the responses everyone.
I am a bit hesitant to conclude that it's a setup issue. In terms of neck relief, a .012 inch feeler gauges slips under a notched straight-edge that is sitting on the board (not the frets), so I think the setup is good. The setup hasn't been changed radically since coming from the NYC store, and it was recently tweaked by Carl Pedigo here in Chicago. But I'm not ruling setup out as a cause. It's low, but not radically so, and seems to fall within the setup guidelines described [Invalid or Expired Link Removed].
It sounds to my ears, like there's something loose on the bass that is rattling when I dig in to the strings with fingers.
I don't seem to have the same issue with my other basses which have similar setups.
I like the sound of fret sizzle too, when I mean to dig in and add a little dirt. But this phenomenon sounds like "thwack-thwack-thwack" every time I hit a normal finger pad-plucked note.