S60 Steering: steering wheel jerking at idle and when turning left

[somervillearron]

hi,
hopefully someone can help me here as this is begining to get dangerous,well i first noticed when im in drive if you put a slight ammount of pressure on the steering wheel with one finger it moves about 1cm left and right on its own back and forth,but now when im going round a roundabout its impossible to steer smoothly but only when turning left you can feel the steering wheel jerking you ever so slightly to the opposite side from where you are turning if you where watching me drive you would think i am moving the steering wheel left to right rapidly but ever so slightly. but i am worried incase it gets worse as i could imagine myself losing control

ive only had the car a week so its new to me,the car tracks straight no vibrations either,i am wondering if it could be caused by the power steering belt? as i am getting a sort of chirp/squeek from the belt/pulley definately not the belt slipping and when you look at the pulley and belt moving it looks like its slightly buckled so am i correct in saying if theres a warped/bent pulley on the belt for the steering then it could cause this?
i had the same issue on vauxhall with a bent pulley

thanks guys