Ooookkkk.

I remember seeing this thread. Now it is happened to me. BIABMeToo smile
Same symptoms.

Noise was not from actual mixer tracks...
I muted all tracks including trough + audio. 9Even tried freezing them) Was hearing just noise throughout track. I tried so many things. Same type of high pitch noise OP posted. I was on version 728. I was hesitant to upgrade, because it was very stable and did not want to invite new bugs for dinner. In any case, I loaded last Oct 8 patch, seemed to resolve the issue. One thing caught my eye...

After update BIAB prompted for new version of Synthfont download. I went to Synthfont site to read about changes. One thing I noticed:

"Version 3.300 could introduce a slight noise on some VST hosts"
Not sure if that was it, but noise seemed to be issue outside of Real Tracks in mixer (as they were frozen and muted)

In any case, I consider my own situation resolved, just posting this in case someone experiences this and tries my solution. Update to latest patch + (and Possibly update synthfont)