Many customers get in touch with us asking why they hear a white noise or hissing sound when they turn up the gain on their Scarlett interface and directly monitor the inputs through either their headphones or monitor speakers when nothing is plugged in. In nearly all cases this is the noise floor that you are hearing and is normal behaviour.
All interfaces will have a noise floor, this is inherent within electrical components as all electronics generate noise. The noise floor is the sum of the noise each component generates. So when you turn the gain up on the Scarlett you are adding gain to the input signal but also adding gain to the noise floor. It will start to become evident after roughly 12 o'clock on the Scarlett interfaces.
This noise will then disappear as you turn the gain dial back to 0. Furthermore this noise will be particularly apparent when there is nothing plugged in to the Scarlett as you are essentially amplifying static as well as the noise floor. For a truer representation of the noise floor alone you need to terminate the input, this can be done by plugging in a condenser microphone (such as the CM25) into one of the inputs but then leaving the phantom power (48V) turned off then adjusting the gain on that channel only. You will then hear the noise floor without any of the static and it should be a lot lower in volume. This is a truer representation of the noise floor.
If you find that you are constantly battling the noise floor in your own recordings then this would suggest that the recording technique is not quite right for the application, either the microphone being used is the wrong type of microphone for the application or perhaps you are recording an instrument level signal into a line level input.
Of course, if you do feel that what you are getting is in fact a fault with the preamp then we would recommend getting in touch with us directly and providing audio examples so that we can listen to them and advise on the best course of action, please don't hesitate to Contact Technical Support.