Increased background noise is usually caused by something controlling your gain. Discord has a habit of changing your microphone's gain directly in windows. Open your microphone properties in windows (windows settings -> system -> sound -> sound control panel, recording tab), and monitor the volume setting in the Levels tab. If you see this changing, then this is what is causing the background noise issue.
If this is the case, then there a few things to try. First is to try setting your default recording device to something not used -- in the recording tab, just select an unused device. Then in discord, specifically select your actual microphone, and run through the same tests to see if the issue persists.
If that still doesn't fix thins, then uncheck all of the extra voice filter options in discord's voice settings (scroll down past the normal sensitivity/threshold settings), and see if things are fixed... and if that fixes it, then re-add filters as necessary and see which one is the cause.