We bought a pair of these headphones for our trip around the world and I haven't regretted it for a second! Since we've been back, I've only used the headphones every few days, stored them carefully and never had any quality problems until now. Recently, however, I have started to hear an unpleasant humming noise in my right ear as soon as I switch on the NC function. I also hear the hum in my left ear, but only now and then and much quieter. The noise is so unpleasant that I can no longer use the NC function. Does anyone know the problem and is there a simple remedy or are the headphones just broken...?