after much messing around I found out it's actually a 3pin fan
I stuck in Low-noise-adapter left over from my noctua fans, think it reduces the fan to run at 9v. but now it spins at 500rpm or so the XT says but the good news it's whisper quite now
hope this helps, also to open it you will need a 2.5mm allen key the front 4 bolts is enough to give you access to the fan / all other headers.
Also in future mods there is space for an extra fan! there seem to be 2 fan headers on the PCB will try and plug in a second fan to see what happens later