Are you sure it's not touching anything/vibrating against something? If it's all hard lines you could install a soft section between the pump and hard lines to stop it from transmitting vibration to the rest of the plumbing. Also take it apart and inspect the impeller and shaft. I had a pump where the impeller had to be set just so or else it would be obnoxiously loud.
I have a pump from Rio and one from Life Guard and they're both nearly silent. I think the life guard was like $15 on bulk reef supply. The Rio was $40 at a LFS. The fan in my AI prime is louder than the Rio, and it's quieter than my laptop.