When new the gast pumps don't rattle, but as they age the rattling is more pronounced as the carbon vanes wear. At some point the pump will lose efficiency due to so much wear and they can be rebuilt with new vanes.
Sometimes the powder from the vanes will cause them to stick and lose pressure, this can be remedied by running some oil free solvent through the pump in a well ventilated area. Watch for sparks and flame sources!
They are always quite noisy even new, they usually have a muffler in the form of a filter with a glass jar over it. You can even add more muffling to it if you are careful not to choke the air supply too much.