It sounds like a rattle, I've had the impeller out and cleaned a few times tonight alone. I did prime it with the output under the water which I now isn't ideal. Would it be worth re doing everything?I would suggest taking a look at the impeller. My old Fluval 2 series used to make a loud noise if it wasn't seated properly. Take it apart and make sure it's clean and in place correctly.
Depending on the sound it could also be some trapped air which is usually released by tilting the filter to dislodge it and allow it to escape.