The silicon etching that occurs during the CVD diamond growth has been investigated as a function of the methane content in the gas phase by SEM and AFM in the tapping mode on pristine Si(lll) surfaces. Size, depth, and angular distributions of the etch pits were recorded. We evidence the strong effect of the carbon content on the etching process. The silicon etching is slightly increased with addition of 0.1-0.25% of methane in the feed gas, and depletes with larger addition of carbon. This etching occurs easily along Si(100) directions. This is explained by the better stability on (100) planes of the precursor SiH2 to remove silane than on (111) planes. A modelization of the process points out the balance between the drop of the atomic hydrogen concentration in the gas phase and the inhibition of hydrogen bulk diffusion into silicon when increasing the methane content and covering the surface with carbon. This explains the occurrence of a maximum of silicon etching. Possible consequences on the diamond nucleation process are then put forward. It is thus expected that the etching of the silicon is generally detrimental as generating low-density silicon surface such as Si(100) where diamond nucleation is inhibited.