How to I control the speed of my CpuFan2?

[CMGCF_ELiTe]

My PC currently has 4 chassis fans, one on the front, one on the back and one top and bottom.
My motherboard only supports 3 sysfans so I plugged the 4th (bottom) fan into my cpufan2 three pin port.
My fan now follows the same rpms as my cpu fan.
how can i manually control this fan to lower it's rpm as it makes a horrible audible noise?
I tend to plug it out or use a cable tie to stop it temporarily (the fan has blue leds which complement my build)
I've tested without the fan and my PC operates perfectly.
All I need now is a short guide (or a long one if you have the time ) on how I can lower it's rpm and ultimately its dB level

 

[Aeacus]

CPU_FAN2 port isn't meant for connecting case fans. It's for those CPU cooling systems that are using 2 fans on heatsink. (Like my Arctic Freezer i32 in my Sylake build with 2x 120mm fans on CPU heatsink. Specs in my sig.)

Rather than starting to mess around in BIOS to control CPU_FAN2 port, buy a Y-splitter and connect 2 case fans on single SYS_FAN port on your MoBo.
amazon: https://www.amazon.com/Noctua-NA-SYC1-Accessory-4-pin-Y-Cables/dp/B00KG8K5CY

With Y-splitter, both fans will run in sync and changing RPM will change RPM on both fans at the same time.