Grain is something that needs to be managed when rendering. If you are getting grainy renders when using the physical renderer in Cinema 4D, it might not be the sampling settings for the render itself. If you have ambient occlusion turned on, that may be the culprit. AO gives you nice contact shadows between all your objects in the scene, so how do you start to fix the grainy Ambient occlusion?
Some quick tips when using Ambient Occlusion in Cinema 4D including dealing with grainy renders
Here, Joren Kandel from the Pixel Lab, shows how you can start to eliminate the grain when using AO, by altering the number of subdivisions that it is using. Altering the subdivision level to a higher number will get rid of the grainy Ambient Occlusion.
It is important to note that Ambient Occlusion is based on scene size. The scale of your objects in the scene will have an affect on the quality of the AO. Joren also provide a quick tip that shows how to best deal with scene scale and AO.