From a similar thread, here a post from @xfilian who had yours problem and resolved it:
« So, a quick update.
I received the ground loop isolator yesterday and tried hooking it up to both the Denon end and the SN3 end. Sadly the hum persists but no problem - I can return the isolator.
I spoke with a Naim technician this morning and he confirmed it is a ground loop hum and that I should try running a copper cable from the ground socket on the rear of the SN3 to the RCA jack on the rear of the Denon where the SM3 plugs into. Will try giving this a go later.«