There are many things that can cause a microphone to have noise and hiss. Different causes happen for different microphone types.
A few examples would be contamination at the high impedance area near the capsule (solder flux or even skin oil from careless handling), a noisy tube or semiconductor, or a capsule that has been contaminated.
Also poor use / gain staging can cause the final recording to have a high amount of noise. Like poor mic placement requiring a ton of gain.
Or a instrument can have a lot of noise and it could have nothing to do with the microphone. Sometimes it is a compromise between tone, capturing the performance in the moment, and a perfect fidelity recording. For example, a steel player is playing a great performance, but the single coil pickup is generating a lot of noise/buzz. Do you stop the great performance to get the perfect fidelity recording that loses the magic of the moment? Or accept the flaws in the fidelity of the recording? If a track were sent to you with high noise, it should have an explanation I would think. Then in mixing (with someone that knows what they are doing) a pro level noise reduction can usually do a pretty good job. A good noise reduction tool with profile the noise spectrum from a moment when the instrument is not playing, and remove that. Not the same as a 'noise' gate, as in a pedal. I would rather have the raw track with the full noise, than a noise gate pedal be added on in the studio.
