Why is the a320 ptu so loud?

The A320 has three independent hydraulic systems, colour-coded green, blue and yellow. Green is driven by a pump on the left engine, yellow by a pump on the right, blue by an electric pump. The power transfer unit is a bidirectional hydraulic motor coupled back to back with a pump: when green pressure falls while yellow is healthy, yellow fluid spins the motor, the pump pressurises green, and no fluid crosses between the two circuits. That separation is the whole point, because a leak in one system must never be allowed to drain another.

The unit is not proportional. It cuts in when the pressure difference between the two systems grows, pressurises the weak side, then switches off as soon as the difference disappears, which happens almost immediately, whereupon the difference returns and it starts again. The result is a rapid on-off cycle, and the sound is exactly what a fixed-displacement pump makes when it slams from no load to full load several times a second. Engineers and crews call it the barking dog.

Location does the rest. The PTU sits in the main landing gear bay, in the centre of the fuselage directly under the cabin floor, in a space that is unpressurised, unlined and open to structure that carries sound straight into the cabin. Passengers meet it most often on the ground during single-engine taxi, when only one engine pump is turning, or when the yellow electric pump drives the cargo doors, and again briefly after take-off during an automatic self-test. It is a normal noise, not a sign of a fault.