I recently went from an old laptop to a gaming computer for my home studio. I installed my version of cubase 4 but now have a hiss or hum behind the recording. Levels show his sound is present even when the mic is unplugged. Can anyone offer suggestions to eliminate it please?
Does this happen when all vst effects/instruments are disabled?
Also… You mentioned a gaming computer. Maybe it has a noisy fan cooling the graphics card or the cpu. Check that, maybe you could disable the fan as a test.
Regards. 
Sound card?