I have noticed the same sound coming from my new Surface Laptop 3, and after some investigation, I have made the following observations:
The buzzing is not affected by load on the CPU, but is consistent while the laptop is on.
Putting the battery on high performance mode does not affect the buzzing in any way.
The sound is coming from directly beneath the keyboard between F12, the power key, and the +/= key.
Pitch can be altered slightly and briefly by pressing the "page up" and "page down" (F11 and F12) keys.
Pressing the power key while the unit is unplugged causes the buzzing to stop immediately.
If the unit is plugged in, the buzzing persists, even if the power key is pressed, which leads me to believe that this is a battery related issue.
Hopefully this can aid in developing some sort of solution. I think this is a fantastic device, and it would be a shame if there were any kind of widespread defect that would warrant a recall, or hardware repairs.
All the best.