The 787 has plenty of hydraulics!
There are two engine-driven hydraulic pumps and four electricly-driven ones. They power three 5000psi systems which connect to Landing Gear, Flaps, Slats, Flight Controls, Nose Gear Steering and Thrust Reversers.
I suspect that what you are hearing are the electrical hydraulic pumps, which run continuously below approx 2000ft during take-off and landing.
The system which the 787 eschews is an engine-bleed-driven pneumatic system (apart from inlet cowl and engine core anti-ice). Air conditioning and pressurisation is provided by four electrically-powered Cabin Air Compressors feeding two packs.
The 787 electrical system is truly awesome. Each engine has two generators; there are two APU generators, three external power sockets, two batteries, a RAT and five engine-driven Permanant Magnet Alternators/Generators. In addition, each primary flight control surface has its own battery pack, to keep them moving in the event of a transient loss of electrical power during abnormal operations.
The electrical system powers engine start, pressurisation, wing ice protection, stab trim and wheel brakes, in addition to the normal services like avionics and lighting. It also provides an alternate power supply for the flaps and slats.