Or should i just replace it? I would need a slim fan (10-12mm), and since only Scythe makes them...Should i go for the 800RPM or 1200 RPM versions (non PWM)? I'm afraid that 800 RPM wouldn't be enough for the job (intel 2600k CPU, stock, in direct air path)
Well you have already the Slipstream pwm running at 900rpm, try running it at 800rpm and check your temps, if you are fine there then go for the 800rpm, also try it at 1200rpm see if you are fine with that. Also check the fan plugged directly to your mobo, see if you can control it, or check if you still hear the buzz without the fan controller.
ps: non-PWM fans can't be controlled from windows, is this right?
Depends on the motherboard, some can change rpms on voltage, check your mobo bios/software.