Weeeellll.... Eheim answered me, with asking me to send the filter to Sweden, avoiding my question. So I sent another message, telling them that they STILL didn't answer my question. This was the response:
we do not tell any db level upon our filters because the feeling from human to human is different. The pump have the normal 50Hz humming. The most important thing is, that you have no air trapped."
I honestly don't know what to say. I think I'll just live with the filter for now. Might swap the pump later if it proves to be ineffective at cleaning the water. Got a pretty big bacterial bloom right now, but I'm sure it'll clear up.
Also, thanks for you guys' help and advice =)