I always wonder about people that say Mac's are silent. I have never in my life seen a system that is totally silent. If system has fans and harddrives it will never be Totally silent.
My iMac is quiet, in fact as it warms it seems to become even better but I wouldn't call it silent.
A humming noise is often a power supply noise so that might be something to look into. could also be something in the area of your system causing feedback, wires next to each other, something not shielded correctly. Only a guess.
I have owned several Apple computers over two decades non all of them quiet, non of them silent. Thats a very subjective term.