04 chevy Silverado 4x4: T/C knocking sound not real loud but loud

04 chevy Silverado 4x4: T/C knocking sound not real loud but loud .. does anyone know what could possibly be making my T/C making a popping noise . I just changed the bearing in it and it's not leaking and just checked my u joints . My chain wasn't lose lose but had just a tad bit of slack . Help please