It is a small unit, not much bigger than a football with high speed rotating parts that we can thank for the noise.
With regard to hydraulics, the largest noise component comes from turbulence in the piping, and in particular, in elbow fittings.
When a large-radius elbow with a relatively smooth bore is used in piping, fluid turbulence is minimised.
However, modern manufacturing methods produces 90° elbow designs that are easier to produce on factory production lines.
These elbows are made from a solid block of L-shaped steel - the outside threads are rolled into the metal, and the fluid passageway is rapidly drilled from two directions, at 90° to each other.
Where the drilling ends, right at the inside corner of the elbow, there is a mismatch in internal surfaces, as regards smooth finish.
This mismatched rough internal passageway surface produces much turbulence in fluids flowing at high speeds and high pressures through the elbow and piping.
Thus you have loud noises being produced, as that turbulence is being created.
The engineers probably didn't even consider the effect on anxious listeners in the back, as they would have disregarded any of this noise as being of no consequence.
The design engineers are more focussed on keeping engine noise levels down, particularly at cruise settings.