The reason your laptop (or any other electrical device for that matter) doesn't electrocute you is that the manufacturers have a vested interest in designing them not to do so, mainly so that they can avoid lawsuits but also so they can continue to sell you more of their devices! Joking aside laptops run on low voltage DC, typically 12 - 18v which is supplied either from the battery or via a mains power supply, the design of the power supply is such that the laptop itself is isolated from the mains and there is no high voltage present. Not all devices need a ground connection in order to be safe, it depends upon the design of the product.
As for your hum problem I wouldn't recommend running ground wires from the USB socket as you suggest, although this is unlikely to cause any damage or hazard it is not a recommended procedure. Without knowing the details of your setup and what you have connected to the computer it is difficult to diagnose the problem but it sounds like a ground loop of some sort. First thing to try would be to see if the problem goes away with the laptop running on it's battery and not connected to the mains. If you can let me know exactly what you have connected to the computer I might be able to help more.
Nice one, I knew that bit about the transformer thing knocking it down from 240 to 12v, this is what im curious about the bit about the design and how we know how high a voltage needs to be before it needs an earth.
Yes the laptop is quiet when running on battery and noisy with the power plug inserted.
Before reading your post I already tried the grounding plug idea and its working ok, totally silent no hum or hiss.
Heres a picture of my setup:
https://pasteboard.co/IoCGENb.png