My first guess would be coil whine.

https://en.wikipedia.org/wiki/Electromagnetically_induced_acoustic_noise

Psygorn (DRILLBIT):
"there you will find a zip file called "F_Concept" if you unzip this file you will find 3 files in it. I double clicked on the file called "F_Concept.stp" obviously MOI3D opens but when I hover mouse cursor over the 3dmodel I hear a noise! it is audible when cursor moves over the model if the cursor stays still there will be no sound! and the sound is like a very low buzzing sound! Could you check and see if you get the same results? (I suggest you to use earphones/head phone) and if yes then what could be the reason?"