Thank you Rowland, that clarifies a few things. I have forgotten about the back of the 450D sensor, it has been a while since I did the cold finger mod.
The Nikon D70 sensor that we are using has a metal back but the back is not grounded. The camera stopped working once I grounded it via a grounded cold finger.
So I can see two possible options:
1. Grounding the cold finger but electrically insulating it from the back of the sensor with electrically non-conductive thermal paste.
2. Electrically connecting the cold finger to the back of the sensor (but not grounding anything).
The 2nd option seems to give lower noise when switching TEC. It is also less dangerous than the first option which relies on the thermal paste making a perfect electrical insulation.
Unfortunately, while we minimised the switching noise it is still present. We are back to leaving the TEC on or off during the frame reads.
Or I can increase the switching frequency to bury the noise inside the image and pretend it is not there
