Can't hear anything on my work speakers, but I don't know if I would hear 9K anyway
But you should look at spectrum on oscilloscope directly from the instrument. Get exact frequency you suspect between 8-9K and look for multiple that could be root of the harmonic. Look for a pattern that might offer some clues. I had a cooling motor on the DM12 causing me line noise. If I run it low speed it generates less line noise.
I assume you tried swapping your headphones/ instruments?
On Audacity to fix the occasional noises that creep in, I have a program that can target (knock out) very narrow specific frequencies. If you can't find the source you might need to go that route.