I was a fleet mechanic and they sent us to a Bendix brake seminar. That guy said that a lot of the time squealing brakes are from tiny pieces of metal getting imbedded in the lining. He said when you machine drums and rotors you need to scrub them good with soap and water to remove any traces of metal particles. That might be true if they squealed right after installation but what about when they start squealing 20,000 miles later. Its either something has gotten into the lining, or the lining is vibrating and causing noise. Noise is cased my movement of the air. Something in the brakes has to be vibrating which is turned into sound. Companywide were forced to use some cheaper brands of lining that were noisy and they didn't last half as long. I also ordered the parts in or shop and I refused to order those crap brake linings. I only stocked Bendix, I had as fleet that wen millions of miles nothing worked better than a Bendix lining. Too bad you can't get them for old cars.
The phone company guys would complain about noisy brakes, and it says in the factory manual that some noise is not a safety concern. I printed that out and gave it to the drivers. We were there to save money, so we were not going to replace perfectly good lining because it squealed after they drove through a stream up to the doors or got mud in the brakes. I could install Bendix shoes and pads. the best brakes made and machine the drums and rotors and in a week those guys had the rotors over-heated and blue and were complaining about noise. Too bad, learn not to abuse the trucks.
The only way to get rid of the noise was with new quality lining and machining the drums and rotors. I did this for the guys that complained about noise that did not abuse their vehicles. The other jerks had to just listen to the squealing they caused. 400 vehicles, I couldn't be replacing squeaky brakes on all of them. 30-years of doing that, literally, thousands of brake jobs. Now I hate doing brakes and pretty much anything on a vehicle.
