I have a Monster Power 3600 unit that's not quite one year old, and is also starting to hum slightly- I believe that unfortunately the cause is most likely the transformer(s) that Monster is using in the units-
Age & heat can cause the metal plates that some transformers are built around to "delaminate", creating just enough of a gap to generate the 60 Hz buzz/hum we're hearing. If this is indeed the case, then then bad news is that it will most likely gradually increase in volume over time-