it is a bit of a process of elimination, 1st the belt (around a fiver), then the tensioner pulley (around a tenner) 3rd would be contaminated fluid (should be red and oily) and 4th could be a failing pump.
but if you havn't followed the service ritual its best to start with the most common culprits and work your way up. changing all of the 1st 3 is less that the diagnostic at most places
