gingerish

hi i have a 96 wrx that used to be auto which was converted to manual prior to me owning car. the box that was fitted had a slight noise when i purchased it but but it has got louder every now n then i get a slight crunch from box whilst driving. not whilst changing gear just general movement. feels like something in the box or maybe diff. i think its about fooked but not sure if rear diff would cause any problems. ie wrong ratio or something. the car in 96 wrx the box is wrx sti ive been told.