Fig 1. Noise Menu

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%.