No camera fault. It is caused my the amplifier mosfets that read out the image from the sensor giving off infrared photons as they operate. These are NOT temperature dependent and thus still happen even if the camera is cooled. The longer the exposure the more impace this constant flow of infrared photons will make. The silly thing is the amp is not needed during exposure only at the end, to read out the data. Thus the 40D turns off the amp during exposure.
I came across this
amp off mod for the 300D. may be ones for other cameras
Scott