R2D2 Mac Pro making sound when CPU usage is high (not fan noise)
It's normal. Many computer components can generate high frequency noise under high load, mostly from power circuits, but fans are usually loud enough to cover them up. The GTX 780 in my desktop does this but I didn't notice it until I installed a liquid cooling system. If you're interested, this has some details of the components that can cause this kind of noise. (one example is piezoelectric flexing of ceramic capacitors causing vibration of the PCB)