That could be backlash or it could be where the table's screwed to the motor mount. There'd have to be a lot of backlash to "feel" it like you say because the maximum width of backlash's travel will be (much) less than one thread turn (1/20th in). You wouldn't want to randomly tighten the leadnut without knowing for sure, and knowing how to tighten it.
Only brass specifically sold as "leaded brass" contain lead, and leaded brass is not a common type you'd find at the hardware store. There are many classes of brass and an infinite number of exact %'s inside the classes. So you need to start with specifying a purpose and work out from there on which type is most suitable and easiest to obtain/afford.