The "hissing" sound noise is a noise of the microphone itself and it is amplified incorrectly or inefficiently. Actally not it is a professional microphone or Primary microhone like wired lavalier microphone,shotgun microphone it will have same situation!
Such as, when you plug a microphone,that time you find the noise is pretty lower with a very low noise and you put the output into the microphone preamplifier, you will have to adjust the voice in bigger , which emphasizes the inherent noise in the signal path.
Other reasons why the microphone may look very low are:
1.Phantom power is not applied when the microphone is needed,Low power warning(sometimes the device can remind you ,but sometimes can't)
2.Poorly microphone cable shielded, resulting in environmental interference and sneaking signal
3.Insert the microphone into the line input of the mixer. The mixer expects a larger signal (line level) and needs to amplify the microphone signal a lot to make it heard... And amplify the noise.
4.After the microphone picks up the sound, the sensitive microphone will amplified each part noise of the equipment, and the recording, amplifying, and recording causes the acoustic and electrical signals to self-stimulate then the hissing noise will be produced. This noise not only affects the audience or the singer, but it can also damage your equipment.
Note: all microphones and signal paths have some noise. If you are recording something very quiet or far away, you need to turn on the microphone preamplifier more, which is inherently more noisy.
There also have several tips to reduce the "hissing noise",you can check out our another article