As Cryrid said, noisemaker plus morph targets are what you should look at. You can even mix noise patterns and adjust intensity with the morph brush and multiple targets this way.
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…