Squeaky coasting and back pedal
It was the freehub; when it is pulled away from the wheel while turning it makes the sound.  I guess putting the chain on the largest cog makes it pull the freehub slightly out. 

I have not solved it yet: this freehub apparently needs a 11 or 12 mm hex key to be removed rather than 10 mm and 10 mm is the biggest I currently own.