Firstly check the level of the fluid. If low this can cause a clicking and a humming when wheels are held on full lock at engine idle. If not think about replacing the pump.
For the TS Astra with hydraulic-electric power steering, it takes a Pentosin CHF fluid. A couple of equivalents I have come across include Castrol Hydraulic System Mineral Oil Plus, and Penrite LDAS fluid.
I would be surprised if the noise was actually coming from the power steering as the engine rotation which drives the steering pump is always the same, sounds like its more likely to be a gearbox issue
If it was ordered with power steering it would have it.
Bad power steering pump
* No power steering fluid in reservoir * The power steering pump is not operating
wheels at angle put most stress on power steering pump, or rack and pinion. low fluid or bad pump, rack and pinion would be your problem.
to my knowledge there is no power steering filter i would check my power steering fluid level
From what I understand, power steering in cars (vehicles) is simply to make steering the car easier. If it were not for power steering, you would have to use a lot more muscle to turn the steering wheel than you use when you have that power steering in place!
If you have an electric power steering system, that means you HAVE NO FLUID. Therefore no fluid to change in the 1st place.
the most likely cause of that would be the power steering pump failing.
could be several places, power steering hose, rack and pinion (or steering gear box) or power steering pump.
No power steering fluid, broken pump, or broken lines.