i've had a slightly different experience w/ my Scythe fans, but i'm also not using the Slipstream 1600rpms. rather i'm using some SY1225SL12L's (800rpm Kaze-Jyuni Slipstream) and some SY1225SL12LM-P's (200-1,300rpm Kaze-Jyuni PWM Slipstream). the 800rpm non-PWM fans are completely silent of course (save for the noise of the airflow itself, which you have to be within inches of the fan to hear anyways), while the PWM fans almost just as silent, even at 1,300rpms. different motor and/or bearing than the 1600rpm Slipstreams perhaps?
regardless, even though i mentioned in a previous post that i'll stick to Scythe fans, it turns out a close friend ordered a few of these Cougar fans and promised to lend me one for testing purposes...so we'll see how they compare to my current fan(s) of choice.
btw Axel, i read in another thread (can't remember when or where) that a certain model of Cougar fan wears out prematurely if used in the horizontal position (blowing air up or down) b/c it uses a bearing that is specifically designed for fans that will oriented vertically (blowing air front to back or vice versa). i have no recollection whether or not this is the fan that was being talked about in that thread...but i just wanted to give you a heads-up since you're planning on using these as top case fans.