First off, ask someone who knows how to check the brake pads for thickness if you don’t have experience, you may be misinterpreting what you are seeing and the pads could appear to be ok, but actually worn out. Worn out brake pads is the most common explanation.
If the brake pads are confirmed to still be well within their wear limit, you may have glazed rotors and/or pads. Trying what @W30post says with the sandpaper is a good idea. If it works though – that’s often a symptom of the pads deteriorating, and you’re probably going to need to replace the pads and possibly have the rotors resurfaced eventually to get rid the the problem once and for all. Ask your mechanic to install the brand of pads recommended by the manufacturers. Some brands of brake pads seem to work better on certain makes of cars.
Another cause of this kind of brake noise – less likely unless the noise starts soon after a pad replacement – is bad or missing shims.