Not sure about "blade rap", but "blade rub" is not unusual on some Case models, stockman knives in particular. Case used to soften the tangs after heat treat prior to stamping the model number, etc on them.. That made them easier to stamp. But it also allowed the blades to flex a bit, especially in combination with the lower hardness to which the blades were treated. The side pressure during opening was enough to bring the blades into contact and they would rub a bit as they opened.
I don't think I cc have a Case stockman which does no exhibit at least some blade rub.