When you have high performance fans [high CFM and static pressure] your fans tend to be pretty noisy [over 20dBA] when you use them at 12v. I will show you how to make a simple 12v to 7v switch so you can silence your fans on demand but not lose the 12v for those high performance moments. Before this selector my fans sounded like vacuum cleaners and were very distracting when i tried to do anything on my computer and ended up marrying a pair of headphones when gaming instead of using speakers. Afterwords I couldnt even hear my fans.

Step 1: Tools You Will Need and Supplies

You really do not need much for this.

Tools
*dikes
*digital multimeter
*crimpers

What you need
*a molex to fan adapter or some time of female molex connector
*a selector switch [single pole double throw if you are going to make one, double pole double throw for 2 in one]
*some terminals

Optional
*a proper molex mod tool
*soldiering iron
*soldier
*heat shrink
*a drill to mount the switch externally
*extra wire

Recommended
*3 way fan splitter

Step 2: Molex Principles

Standard Molex wires are 12v [yellow], ground 1 [black], ground 2 [black], 5v [red]

fans typically run off of 12v, but if you use 12v positive and connect the 5v to the negative it acts like this 12v + [-5v] = 7v. The 5v is used as a negative voltage in this case which results in 7v. This is pretty basic stuff. What I'll show you how to do is make a selector switch between 12v and 7v. There is another post on here for a 12v to 5v selector switch, but 5v is almost useless, doesnt have a high enough CFM and static pressure to do any good and some fans wont even spin up at 5v.

Step 3: Taking Pins Out of a Molex

I didnt have a molex tool, but i found that something thin and small [in my case i used a probe of a multi meter] to push the small pins in so you can remove the molex plugs

http://youtu.be/TcV7l1pER2M

This video might help. All need to be removed except the 5v, the ground [black] closest to the 5v [red] should be permanently removed because it is not need. The 12v [yellow] and the ground [black] both need to have a wire cut off

Step 4: Cutting

The wire that DOES NOT go to the 3 pin fan connector on the 12v pin needs to be removed. The opposite is for the ground, the wire that goes to the 3 pin connector fan is cut and the other wire is left. At this point you should modify cord lengths so you can put the switch were you need it and go ahead and drill the hole for your switch

Step 5: Connect the Wires

Put the pins back were they are supposed to go on the female molex connector. The 5v [red] goes to one side of the selector, the ground goes to the other side of the selector then the negative part of the 3 pin fan connector goes between

Step 6: You Are Ready to Go

Hope this helped you.

There are other ways you can do this like the 12v to 5v switch here:
https://www.instructables.com/id/DIY-PC-Fan-Controller/#intro

I have already stated why 5v is not a good idea

You connect multiple 3 pin fan connectors like he did, personally I use a 3 way splitter

You can also make a 7v only adapter which is what I use in on my computer

You can also make another thing that is shown below