From the sound of things it defitently seems like the cd player is at fault here. If you can, try using a different cd player or dvdp or vcr, whatever for the source.... Actually...

If you turn your system on and leave the cdp turned off and try turning the volume up. and the HUMMM is not there. its the cdp. If it is, its probbly the NAD. i dont know if that nad has them but make sure you have the shorting plugs in the tape input..

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