Anyone Know About Eheim External Filters? Mine Is Really Noisy

I have just bought an Eheim 2213 filter but it is quite noisy. I have checked the impeller and it does not rotate but oscillates (if that's how you spell it?) It looks like it is supposed to do this despite the instructions stating the blades should rotate freely. Does anyone have one of these who can advise me? I have tried tilting it to get rid of any air. Thanks