If it's that banging noise I heard whenever Z axis reverses directions and it's not way covers banging then it's probably extreme backlash. Most likely bad or loose TAC bearings on the ballscrew ends. Very easy to replace. Make sure you install with correct configuration.
Check your ballscrew and if it's oily and not galled then the ballnut should be OK. That would be your greatest expense. The rest is just bearings, couplings and mounting hardware to check. The biggest pain will be pulling back the Z axis way covers. If it comes to that, be sure to adjust the Z axis gibs while you have the covers back.