Phil. Cutting through the previous BS - most modern windows have a strip locking system with cams and shootbolts. If your windows have cams that engage in keeps within the rebate, open the window and look at the gold lock strip and on each cam you will notice a small mark or line. This could be facing at any point on the compass. By inserting an allen key into it you can turn it through 180 degrees, and as it moves eccentrically around its centre, it will have the effect of tightening the sash into the frame when you close the window with the handle. If its too difficult to operate at 180 try less rotation and see if this works. Finally, in my years of fitting and snagging pvc I have found that it is not always the opener that can cause noise - check for pinholes or mis-seals in the silicone around the frame and reseal if necessary. Good Luck.
Kev@KL Plastics, November 2008