Bloke at work had similar on his 1.8 mk1 focus. Had a little leak somewhere and the pump was noisy. Topped the fluid back up to maximum and the noise went away, but came back when the level dropped again. Chucked in some PAS system stop leak which cured the leak then with the level full it stopped making a noise. The leak didn't reappear for the next couple of years he had the car.