I'm a bit confused to be honest. Sculpting anything generally means raising or lowering the surface. No surface change, no noise.
If you didn't want to change the surface, you could try turning off zsub/add and just paint the noise with RGB. Or use one of the bump materials which would basically convert the polypaint into a bump map.
Otherwise you'd have two options for noise that both affect the surface. One is the NoiseMaker Plugin (a low strength will keep the surface from changing too much while adding the noise, and you can use masks to control it). The second is the Surface Noise brush Modifier. It uses similar settings to the noise maker plugin now, but the important thing to note is that it's a brush modifier. If you use it with the Standard brush, then the brush is going to act like a normal standard brush, just with noise added. So to prevent large changes to a surface, you could try it on the layer brush (perhaps with a morph target before sculpting), or on the noise brush (which shouldn't alter the form too much).
Last edited by cryrid; 04-10-2012 at 01:15 PM..