Also check to see if there is a back pressure issue. Just remove the airline from the pump while the pump is running and making the louder noise and see if that makes it stop. If it does, you need to install a valve within the line to bleed off that excess air. Back pressure causes the diaphragms inside to run hotter and wear out faster. If the diaphragm needs replacing, do the pressure test after replacing it to help keep the new one from wearing out faster.