vibrating steering wheel at standstill - power steering fault?

[SilverSaloon]

hi

my wife's Citroen Picasso has started to develop a vibration when you turn the steering - even at idle.

so you can sit in the car and slowly turn the steering wheel and occasionally you feel a wobbily feeling through the wheel.

i topped up the power steering fluid last night as it was low but not too low.

still the same.

does anyone have any ideas before i take it to a garage?

thanks

derek