(If I'm right my post will appear "before the original" question.) BMW still makes plastic impeller water pumps. They had a bad run back in the mid 90's of plastic impellers that would break prematurely but that is apparently "fixed." Anyway there's a good chance yours is plastic but don't worry and replace it as normal. Or replace it now if you like they are pretty cheap, but if you do that go ahead and do the radiator, expansion tank and hoses. They are more likely to fail before a plastic impeller.