u got how many fans in ur CM690 ? i also using CM690, i also annoyed by these kind noise n i solve it by removing the front panel steel bars, those grey in color ( dunno wat is the exact name for it ) n i also remove 2 of my top exhaust fans. then the noise gone.

i've been told tat top fans r useless if u r using 120mm 90CFM coz the honey combs holes r too small so the heat cant b blown out. n yes it is true.