Something that adds noise to a percept, or does not.

Mirrors tbp/monty/frameworks/models/sensor_modules.py::MessageNoise, the protocol a sensor module's noise layer answers.

Monty passes a numpy RandomState that the noise mutates as it draws. Here the stream is an argument instead: a noise layer draws from it and answers a percept. It is the episode's one stream (Monty.Random.Stream), the same one the motor system and the goal generators draw from, because it is the same one in Monty — so the draws a noisy percept makes move the numbers of everything stepped after it (docs/monty-differences.md §2.1).