Ford Mondeo faint whistleing

If it is a concern , you could change p.a.s fluid and see if that helps (make sure you get right fluid ) Otherwise live with it unless it gets worse .As for noise , it could be a disc pad slightly sticking in calliper.Again if bothering you , remove pads and clean out calliper . Grease back of pads and contact points in calliper with Copperslip grease and refit .Could check there is no movement / play at that wheel which is making whistling noise . Sometimes you can get slight whistle when extra load put on wheel on cornering .Usually not worth bothering about .Hope this helps.