Unfortunately, there is not a simple way to eliminate all noise / flickering, since it depends on the render engine and it may have multiple causes (GI, AA sampling,...). Please feel free to take a look at the following link. It includes useful information regarding this topic and some tricks:http://forum.itoosoft.com/index.php?topic=905.0
In case you're using the V-Ray renderer, I would suggest you check / ask on the ChaosGroup forum. Many of their members are experienced users of Forest Pack, and they can suggest you other useful settings.
Hope that helps a bit.