The BSE benchmark Sensex was down by nearly 539 points in the noon trade today. The 30-share barometer, which opened on lower note in the morning, was further down by 539 points or 3.36 per cent and was trading at 15,526 during noon.The wide-based National stock Exchange Nifty index also fell by 146 points, or 3.04 per cent, to 4,666.