How long have you had your C series?
I think it's luck of the draw ..... which is still inexcusable.
My second C3 came silent out of the box.
My C4 is extremely quiet but not silent.
While these filters do a great job, I find it hard to believe Fluval doesn't test them in water before they leave the factory ... and/or work a litter harder on the impeller design to achieve consistently silent results.
You might give their customer service department a call and see what they'll do for you.
It's not just Fluval C series. There seem to be more noisy AquaClear youtube videos than Fluval C series. For example:
And this guy's experience was identical to mine: