If they hum or vibrate when you apply brake pressure you might have slightly warped rotors, if your mechanic is sure that the rotors don't need to be turned then I would start with a vacuum bleed. If you have a buddy to help do a quick bleed process first then a vacuum bleed if its still making weird noises. Some noise can be also be caused by contaminants between the pads and the rotors. Pull the wheels/calipers and get the rotors super clean. check for any grease or oil on your pads and if there is swap the pads, they aren't usually worth the hassle of trying to clean. Be extra careful with brake cleaner spray, the cans are usually really over pressurized and the spray is caustic and extremely flammable. once you have them completely cleaned put everything back together and use a bit of brake silencer on the back of your pads, If they still make noise after thay are super clean with a fresh bleed and new pads then have your mechanic turn your rotors cause they are probably a bit warped.
Hope that helps
13 WRX Hatch - Cobb OTS Stage 1