Thanks fnc for your info. We had our from brakes pads replaced by dealer at 30k miles and we never had this problem at that time.
But there is no noise when there is slow braking. The noise only develops when there is semi-hard braking on or about 40-50 mph. It sounds like a low abrasive noise. There is no sharp or high pitch grinding noise like when your brakes wear off.
My mechanics have taken the brakes apart and they've used sand paper to make the pads smoother. After this procedure, brakes have reduced the noise but still make annoying noise. Do they need to replace with a new sets of pads?