Crawfish actually do rattle when they are in the back peddling mode, their claws clang together...tickling the lateral lines of the predators.
We had this discussion a while back about the Okeechobee bass being conditioned to lures. FishingWrench brought up a study conducted by the late great
Doug Hannon I think. The study showed the bass could remember a lure by an "acoustic footprint" and equate that lure to danger. Don't know how long their memory lasted but it adds credence to your statement about some variable turning off the fish. But I've also heard from more than one person about watching smallmouth while diving that when a lure bounces off the bottom it produces a debris cloud or a big "ca-motion" as aarchdale says. The divers witnessed smallmouth coming down to within a couple of inches of the lure and flaring their gills to taste the debris cloud without touching the lure at all. I can't imagine the fish would be turned off by a crawfish scent at that point. This behavior to me would indicate they are in a neutral feeding mode. So, having said that, I think scents will work more than not.