difference maps in zbrush

[Loud_noises]

Hi, does anyone know of a way to create a bump/displacement map in zbrush that is difference map between two objects. I want to bake a map on a hard edge model that only outputs the sculpt not the edges for example between an untouch level 6 model and a level 6 version of the same model that has had surface noise added. Or is there another way of avoiding having the sharp edges and recesses added to the map?