I'm just disappointed that the OP stole the opportunity for someone to do the old "humming because it didn't know the words" quip...
On a slightly more serious "note" (!), GFCIs have balance detecting transformers inside them, and its conceiveable (or maybe even likely) that there's the source of the humming, but I'd have thouight it would only occur under load, and given I've never heard one of these things hum, I'd wonder what sort of load could induce a balance transformer to make a racket...?