Feather #3
The hydraulic pump noise you can hear is the Centre Demand Pump, it operates when,
System PX is low
on the ground for the first 3 min after the second engine is started, or
from takeoff thrust set until 2000' is exceeded,
flaps or slats in motion, or
during descent from when the gear lever is down and RA is less than 2000' until gs is less than 60kts.
There are 2 electric motor driven pumps to provide centre hydraulic pressure, they alternate between being primary pump and demand pump. Depending on the sector you may not hear the same hydraulic noises.