Ensure that all volume sliders in your operating system, volume mixer and music software are set to 100% and that there's no output limiter enabled.
If everything seems to be OK on the software side of things the problem is most likely that your current amplifier is not powerful enough to output the power and current the headphone requires to operate correctly. You can read more about amplification here.
The solution: Fix your system settings, or get an amplifier with more output power for your headphones
When two or more devices are connected to a common ground through different paths, ground path noise, or a ground loop can occur. Thus, a system grounded at two different points, with a potential difference between the two grounds can cause unwanted noise voltage in the circuit paths. Currents flow through these multiple paths and develop voltages which can cause damage, noise or 50Hz/60Hz hum in audio or video equipment.
Solutions:
A constant audible stream of hissing and crackling is a problem that mostly PC users face. The noise is called Electro Magnetic Interference and is caused by the multitude of electronic components on a motherboard's south-bridge causing the on-board audio codec to glitch. Motherboard manufacturers are trying to combat interference by shielding the audio components from disruptive interference, but if you want to effectively combat this problem there's only one fix; not processing audio inside the noisy PC.
The solution: Buy an external DAC, or DAC/Amp combo, that converts and amplifies the audio signals outside of the "polluted" PC environment.
This issue with analog volume knobs is commonly referred to as a "dirty pot". Dust and other particles can get into the housing and cause intermittants at the coil contact of the slider block and the resistance element.
If you want to know more about the inner workings and of a "pot" and how it operates check out this video.
The solution: Unplug your headphones, turn off your amp and disconnect it from power. Then repeat to turn the knob from 0-100% for 20 times or more. This will cause the wipers to travel through the entire element and clean out the particles. If that doesn't fix the issue you need to open up your amplifier and try to lubricate and clean the pot carefully with contact cleaner
This is the bane of analog volume knobs. There's no way to fix channel imbalance at very low volume settings because the wipers physically cannot be in the same space.
Solution: Lower the gain setting on your amplifier if possible. Otherwise reduce digital volume and increase analog volume. If that doesn't fix it...shit's broken.