Hi
Thanks for the links. It looks like a TRRS jack works with the dock, but a standard stereo jack causes the hissing.
I suspect Kensington have fitted a TRRS jack and when you plug in your headphones, it is grounding the mic, which causes the hissing. When you use the TRRS to audio and mic splitter it works correctly.
A good explanation of the jack types is here:
http://www.cablechick.com.au/blog/un...d-audio-jacks/
Wim