A 6255 will have some noise, it is spinning considerably faster and db is not the issue with the pumps, it is pitch or frequency. The db is at or below normal living room background volume, the higher the speed the higher the frequency, the more annoying the noise even though the volume is not changed.
To dwest:
1) Make sure the front cover is secure, the gap is even on all sides. I have seen pumps that were crushed slightly, then the housing is a real struggle to get on besides just the cable grommet, the prop can then rub the housing as it is elliptical from shipping damage. Make sure the propeller is undamaged, a chip or worn end results in imbalance and a lot of noise.
2) The pumps do wear in and seat, we generally advise that noise is reduced 50% after 4 weeks and a further 50% in another 4 weeks. After 8 weeks you should have 25% of the noise when you started. This is because in running the surfaces are polished and seat together. The first few days can be the noisiest from air trapped in the pump, this can be solved immediately by assembling the pump under water, install the drive unit in water. The sweet spot is 60-80% speed, this tends to get a faster wear in, with a lower noise frequency.
3) If there is no improvement, we want to test the pump held in hand to rule out any vibration transmitting through the mounts or cables, the only moving part is the propeller assembly (drive unit) a replacement may be needed to solve it if it has some inherent imbalance.