I got the exact same problem, I've purchase my surface laptop 3 blue cobalt in replacement of my macbook air ( i regret so much). Since November i got 3 differents devices, and all had the same problem this faint electric noise.
The support recommends that I send it back for the 4th time.
Someone does have a unit without this issue ?
I have a Surface Book 3, Surface Laptop 3 13.5 and Surface Laptop 3 15 all of them have then same problems with the faint electric noise and also the Bluetooth drops out and mouse stops responding. I am Very Disgusted with these Products that I have spent over $5000.00 for.