Squealing brakes

Since changing brake pads and discs at same time they have started squealing when brakes applied gently. When brakes are applied with more force this does not seem to happen, tried to get this fixed on a few occasions with different mechanics and have been told they can find no fault. Changed perfectly good brake pads in desperation, fault went away for a while then came back. Had brakes looked at recently, cleaned etc and to my horror squeal is now there when going around bends and occasionally when driving straight ahead without applying the brakes noone can tell me what the problem is, please help!!