Can I reduce the voltage on the 5v-pin on my RPi2?
I have a fan conncected to pin 4 and 6 but it is to loud.
Is it possible?
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No but if you want a lower voltage use the 3.3V pin! – King Balloonion Jan 12 '17 at 17:56
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Disconnect the fan - it is not needed. – Milliways Jan 12 '17 at 22:48
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No, you cannot... and, well, you should not.
The 5V available at those pins is directly connected (via a protective circuit) to the 5V "power" micro-USB connector. The Pi has no way to change that voltage on its own.
Technically you could apply a lower voltage there at the power connector by using a different power supply and thus get a lower voltage at the Pi's pin header too. It is however not adviseable to do so as USB connected peripherals might not work properly (they get this 5V too) and you might undervolt the Pi with all the nasty consequences. (Learn more about powering the Pi at our blog.)
The most simple way to "tune down" your fan a little is a series resistor between the pin header of the Pi and the fan itself. Make sure the fan still spins properly though and select an appropriate power rating of that resistor.
Edit (re comment): Pins 1 and 17 are connected to the 3.3V rail. That voltage is generated onboard from the 5V by a voltage regulator. While it drives the Pi itself it can be used to power peripheral devices from the GPIO header. Whether that is save depends on the fan (note that a 5V fan does not necessarily run at 3.3V). Above linked blog post states:
The surplus power from the 3.3V rail is more limited than what the 5V rail can deliver. We can estimate 3.3V surplus power by taking the rating of the converter (1A, per spec) and subtracting the load required by essential power consumers like the SoC and any GPIO signal connections. Of course, the surplus power available will be lower when the Pi is running processor-intensive workloads and multiple signal processing applications. But in many cases there should be sufficient power for low-power sensors or devices which operate at or near 3.3V.
