Though I've come across it numerous times, I only today questioned the physical origin of the seemingly ubiquitous 1/f noise. I was both surprised and excited to learn that there is, as of now, no general theory of 1/f noise; it has been applied to signals of various types but eludes an interpretation outside of the signal's source.
In contrast, white noise and Brownian noise have well understood theories and are related through an integration/differentiation.
There are some references to 1/f noise and its relation to self-organized criticality, but I'm going to refrain on making a comment about this until I read more.