Engine pinging

I have a 2001 Toyota Corolla with 128,000 miles. It recently started pinging when I accelerate anytime over 50 mph when the outside temp gets above 85, the warmer it gets, the louder it gets. I have replaced the spark plugs (NGK Iridium). I do not have an EGR, the timing is controlled by the ECM and is not user adjustable and each cylinder has its own coil. I have changed brands and grade of gas with no change. The only sensor I can't check easily is the knock sensor(remove intake manifold) and it costs between $160 and $200. Any ideas would be greatly appreciated!