Alright, that title is horrible description for what I'm about to explain but I can't think of a better one. I will try to explain it the best I can and I include a video of what it does below.
Now I'm a very firm believer in if it isn't broken, don't mess with it! I don't experience any problems, so I just consider this some kind of "feature". I have no freezes or crashes. It hasn't BSOD ever. No heating/cooling problems even with Battlefield 3 running or rendering videos. I also don't see any critical errors in Windows Event Viewer.
When I turn my PC on it is like it starts up in a lower power state. Everything boots fine like normal, I enter my password at the login screen. Then as Windows loads and begins to display my desktop it is like my computer goes from a low power state, spins down almost like its turning off briefly but then spins back up. You can kinda hear it when I put the camera up by the case. After that, everything runs flawlessly.
Is this something that is normal? I've never seen this before with the other two PCs I've built. I have double checked the wiring and it is all correct. The powersupply is within proper Volts. I have noticed that apparently my motherboard has a plethora of energy/power saving features built into it. I don't know what my settings in my BIOS are at the moment but will check next time I reboot. Could it actually be a feature of my motherboard that it has some kind of lower power phase when it is booting up? Like these Quiet/Energy Saving/Power Saving modes I'm seeing in my manual?
ASUS Sabertooth 990fx
8gb G.Skill Ripjaws
ATI Radeon HD5870
Your power supply has fanless mode in low load, if you load your system, it will need cool down your psu, that's why your psu is not spinnig the fan in low load, you can browse youtube and look to the corsair psu's they have this feature.