I broke my chain catcher, so i replaced it. but after assembling everything back on to the bike, i noticed that there's some rattling noises in the bottom bracket. Does that mean that i should buy a new bottom bracket?
Maybe there is a problem with the bottom bracket, maybe you re-assembled something incorrectly, maybe there is something else loose that is rattling.
Check the bike for rattles and loose components by holding it by 10-15cm above the ground and dropping in onto its tires.
Check the bottom bracket is re-assembled correctly.
Turn the cranks by hand. They should feel smooth with no 'notchiness' or roughness. Try to move the crank axle in the bottom bracket shell, there should not be any movement or play.