In this case, a client reported a lot of noise in the shadows of his forest of opacity-mapped trees. In a progressive render, he saw lots of fireflies at the low AA levels, so it seemed to be something out of the ordinary, since he’d never had this problem before in other, similar scenes.
He managed the get rid of the noise by cranking up the light samples, the AA, and the transparency depth, at the cost of extremely long render times.
But sampling wasn’t the problem, or the solution, in this case. The problem was a large “sky” sphere that had the skydome HDR mapped to it. This sphere was just there to make the sky visible, but this sphere was visible to all ray types. The solution was to make the sphere visible to camera rays only.
Unlike a Skydome light, a textured sphere isn’t importance-sampled intelligently by Arnold. So you’ll get noise and fireflies from the random diffuse rays that happen to hit a super bright pixel in the sky texture.
hat tip: TI