Recently my Harman Kardon AVR510 receiver started "humming" pretty badly... actually the sound is not from the unit, but can be heard in every speaker, even with the volume completely down. I use monster cable wires, and have it connected to B&W Nautilus speakers. I have checked the surge supressor, and have even tried plugging the receiver directly into the wall. Have tried different outlets. I have removed every single connection to the back of the receiver and tried running on FM, but still have the problem. I have disconnected the speakers one by one, and it still does it no matter how many or which speaker is connected. This only started recently. The sound goes away for a split second when I switch inputs, and then comes right back. Doesn't matter if it is digital or analog input either. I guess I will have to take it for repair, but thought I would inquire here first to see if I am missing anything, or to see if HKs are known for this... any input would be appreciated. thanks