Thanks for the post and sorry you seem to be having some issues.
Can I ask how you've narrowed down the buzz to just on two ADAT channels? Are they metering in Mix Control?
If you connect a TOSLINK cable, does the buzzing on those ADAT channels continue?
When you select 96k within Mix Control, can you check that Windows is also following that sample rate change by navigating to:
Control Panel -> Sound -> Playback/Recording tabs -> right click 18i20 -> Properties -> Advanced -> drop down menu.
If you match the Windows sample-rate with Mix Control and that of any sessions you may have open, does this effect the buzz you receive?
What version of Windows are you using, as well as what version of Mix Control?
Please also note that the audio attachment in your post doesn't seem to be working, so I have been unable to take a listen.
You may wish to submit this information as an email case to us via:
Simon // Focusrite Technical Support