The latest versions of V-Ray, in particular in the third version (about 2.4 just do not know, did not set), a new optimization algorithm miscalculation scenes with a large number of CMV light sources. The gist of it is that when it is activated, then for each pixel in the final rendering stops rendering the illumination of each light source in the scene . Instead, turn on adaptability and new algorithm renderer randomly decides which light sources are taken into account in the calculation of the color (light) of the pixel, and which are not, that is sampled, not all lights, and some random number (which it defines adaptive mechanism this algorithm). Sempliruya (calculating) the illumination of two adjacent pixels different number of lights o can be taken into account, so that even the neighboring pixels to render can be very different in brightness that you see, and as a rash or noise.
The name of this new algorithm - Probabilistic lights . (Probabilistic in English -. "Probabilistic"😉
It is activated only if the scene includes a light source is installed in more than his only setting. By default, it is just 8 lights o. For this reason, making the 9th light, you see the noise as «Probabilistic lights» included in the work.
Take away the 9th light source and meaning «Probabilistic lights» Choose a smaller 8u. Most likely you will get the same noise.