The Noise tool does the opposite of smoothing. Normally 3D scans have a bit of noise in them, but filled holes are perfectly smooth, so the noise brush can be used to make the filled areas look more like real data.
Fig 1 shows you what you get when you use the <3> key; there's only two options:
<3> - Painting
Press and hold the <3> key to apply noise at the mouse pointer location. You can adjust the degree of noisiness under the Brushes panel.
<G> - Noise Marked Area
Fig 2. Noise Marked
Or you can use the <G> key to mark an area out first, then use <3><G> to apply noise to that area.
First you will be asked for a Border value; this is the area outside of the marked faces that is used to blend in the noise texture (see Fig 2).
Next you'll be prompted for the noise amount; the value here is a percentage of the size of the faces being affected. For example, if a face is 1mm long, then a noise amount of 100% will add up to 1mm of noise to the vertexes of that face. A typical setting here would be around 10%.
