1 13mm bolt driver side which has a big toothed wheel round the outside, another 13mm bolt passenger side thru a slotted hole, and a 15mm iirc bolt where the pump actually pivots. loosen all 3, then use a large adjustable spanner on the toothed wheel to set tension. want around 3-5mm deflection on the belt. if that doesn't help, replace it. might as well to alt belt while you're there.