Strange things happen.
I swapped the Fractal Design PSU with the 750W Thermaltake in my older PC, because the other PC is louder the sound doesn't bother me as much. The other PC is a few metres behind me when I'm at my main PC (which is on my desk and to my right).
When the old PC was running while I was at my main PC, I noticed a faint buzzing sound from my main PC, sounds similar to the noise from the Fractal Design PSU but much quieter and doesn't go away under load.
I'm not sure where it's from though as it doesn't get louder when I put my ear closer to the PSU. The two PCs are running off different power sockets so I have no idea why this is.