growl?

So warwicks are known for their "growl" (especially the thumb), but I want to know is what causes that sound, and why it doesn't sound, say like a jazz bass, I mean they've got the same pickups, what separates a Thumb BO4 from an American Professional Jazz.

Is it possible to have zero left hand noise when playing pizz? Is this a matter of getting the scoop/camper right on a fingerboard? When I left my fingers I get a clank/string noise when pizzing that drives me nuts. I have zero left hand string noise when using the bow. It shows up mostly when...