as per coops i would check in the following order:
spray wd40 on the belt, does the squeel go?? if so it'll be a belt problem.
1) check tension of aux belt- adjust accordingly
2) check the state of the aux belt- replace if it seems to be split on the tracks
PAS faults:
1) bleed the PAS fluid, remove cap and turn your wheels lock to lock several times untill the air/bubbles stop surfacing in the PAS res
2) Carry out a PAS fluid change then bleed
3)check for PAS fluid levels and leaks.
allways get the bubbles the slick 50 cured it but get the bubbles when cold. even when bleeding it out wore me dam tyres out lol
replace the unit
hope that helps lol