If none of these things work out, see if you can take it back. I remember seeing on the one I just picked up a few weeks ago that they have a 3 year warranty on them. It would be interesting to see if they would honour this for a noisy impeller.
I'm pretty sure they the AquaClear impellers only fit in one way, with the magnet down. I'll try lubricating the impeller with petroleum jelly. If I can't get it to stop rattling, I'll email the manufacturer and ask if they can send me a replacement impeller. I don't want to return the entire filter, since I already got it going and it's on a tank that has fish in it.