I have brought MSI 790FX-GD70 with AMD phenom 965 3.4ghz processor , after assembling the parts when i switch on the system then the fan of the AMD processor creates sound just like it is overloaded in normal condition [ just like when 3d application renders a image or a movie or when playing games ] the speed of that processor fan is about 6900 RPM , is there any setting in BIOS for the motherboard to control or to reduce the speed or noise of that fan .
That depends really on your BIOS menu options. AFAIK on some you can set a max speed. Have you tried going through them?
In any case, I wouldn't recommend lowering the speed of your current fan. If you want to reduce noise levels get a fan that provides the same cooling but runs quieter, or consider something like water-cooling. The problem is that if you lower your fan speed, your components might overheat, something you definitely don't want.