most of the time, they are either just not lined up properly, or have some grit in them... I'd recommend pulling it apart, & cleaning out the magnetic well the impeller base sets in ( it's magnetic, so any grit with iron content won't just come out ) if everything looks good, it may have a manufacturing burr, or deformity...
when setting up my newest Aquaclear... the base must be fitted with an o-ring into the filter housing, when I started it, from new, it was also noisy... I was able to wiggle the base slightly, which must have helped it line up better, as it quieted up right after...