MSI Z370 SLI Plus. Also on MSI P67 GD65. I expect it happens on others too.
Background noise in headphones and speakers. Noise changes with cpu usage. Changes with mouse movement. Noise at constant level. Does not change with volume adjustment. Sound like hard drive sound. After reading numerous forums and trying many fixes I removed motherboard and checked for standoffs in wrong place contacting motherboard. I found that they were all installed correctly.
The fix was to remove the input/output shield. This is the chrome plate that snaps into case that the motherboard rear connectors protrude through. I found no bent fingers touching where they shouldn't.
Hope this helps someone.

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