If you define

$$Discretise_n\(x\) = \{ \frac{m}{n} | \min_{m \in \mathbb{Z}}|x - \frac{m}{n}| \}$$

then you can calculate

$$Dissonance \(x\) = \sum_{j=1}^{\infty} \( x - Discretise_j \(x\) \)^2$$

as a good metric of the amount of dissonance two notes with a ratio of x in frequency will cause. If you look at the graph of this function, you will see it has fractal qualities and several local maxima. The maxima are good starting points for generating strongly dissonant noises, if you are into the noise/japanoise/extreme noise genres (ala Merzbow, Massona, Government Alpha, etc.). I have used this to good effect in my own projects.