mag pumps are normally extremely quite, i have 3 on my tank and you have to touch them to know they are running, one of them is even plumbed externally on my calcium reactor. i think you have one of two things going on, either the pump has some air trapped in it making it rattle or you have a bad pump. try unplugging the pump several times in a row with a few seconds between each restart, if theres any air trapped in there that should help get it out.