We will list, in order of probability, the eventual causes that could create these fastidious noise:Alignment of the pads with the disc: Unscrew both the bolts that hold the brake calliper to the frame (usually with a allen nº5). When they’re undone, pull the brakes till the end. At this stage, whilst holding them, screw back in the bolts. At this stage, you will have aligned the pads with the disc. Bended brake disc: Its a very hard work with a doubtful result to straighten it, usually the best solution is to change it with a new one. But if it occurs during a ride, and therefore impossible to change, turn the wheel slowly in order to detect which part of the disc gets into contact with pad.Another way to detect the problem, is to dismount the disc and put it on a flat surface. In both cases we will try to straighten them using a lot of care (since they can break easily). Dirt: It doesn’t often occur that the noise is due to the dirt in the discs. But I could happen. In this case, they sell specific products for the cleaning. If at the time, you can’t get hold of them, a very efficient product is normal pure alcohol. But if the dirt is deposited on the pads, the best solution is to change them or, temporarly clean them with a rasp. Stuck brake piston: (They are the small cylinders that push the hydraulic oil when we pull the brakes). When we leave the handles, if the piston doesn’t go back to its initial position the brakes remain pulled, therefore rubbing on the discs and creating disturbance. This could be due to some dirt or lack of oil.If they doen’t return to the original position we will dismount the pads, with the use of a screwdriver or something similar try and push back the piston to its original position and in the meantime spray it with some lubricant in order to make it slide and do its work properly.
Important: Be very careful not to wet the pads and the discs with the lubricant (best to wait till it dries).Sometimes you may hear some noise when your tabs are new. In this case you have to try them out a little in order to remove the initial layer that cause the noise.