I wouldn't fixate on the fan speed but just keep track of temps and when and if they rise them clean all fan blades and blow out fins on cooler. Glad you're pleased with the new addition, even if I could talk you into a H series water! lol
If temps are ok then I wouldn't stress on the fan speed either. If the warning is from AI Suite II when Windows boots, then you could check AI Suite settings for a fan alarm threshold, although in PC Probe that I have the lowest alarm threshold is 600rpm so the alarm would still trigger.
You could also check for a fan speed setting and set the minimum speed just above what the alarm wants, in my case I would have to set it to 610 - 620 depending on fluctuations. I have AI Suite, not AI Suite II and it includes Fan Expert which allows me to control the speed to temp ratio.
If it's the BIOS the produces the warning then I'm not sure if you can stop it, but if it doesn't make you press a key to continue then don't worry about it.
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