2006 Range Rover Sqealing Brakes

I bought this vehicle a couple of months ago. In this last month the brakes have started to squeal and extremely loud. My first thought was it was a brake wear bar and I called the local dealer and was told this model doesn't have a wear squeal bar, but I should bring it in so they can take a look. So I went to the dealer, and a tech took it for a drive, and did a short visual all in my presense and said the brakes are in great shape but my rotors needed to be turned, and quoted me $400 for the job. I wanted a second opinion so I took it to a local brake shop(Midas) and they did a complete brake inspection, they told me the same thing that my brakes were in great condition but my rotors needed to be turned. And at a cost of $400 also. I took my car to a local garage(kind of like a car cub) and rented a bay and I removed the rotors myself and took them in to get turned.(Through my job we have access to repair equipment at a fraction and at times no cost) it is excellent equipment And thinking I was cutting cost that way, but within a day the loud squealing came right back. While I had the rotors off, I did rough up the pads with a little sand paper, I didn't spend to much time on this since I didn't want to go overboard. The pads have a lot of life left in them, what else could be causing this squeal?