Ok, so it is being generated by the Denon.

Not un-common in any electrical bit of kit, the sound you are hearing will be something like a failed or dry joint in the soldering or an individual component (resistor or the like) starting to break down and of course if this is on the pre-amp side, then the noise in amplified hence you can hear it through the speakers.

So, I am afraid it is either find someone to attempt a repair or replace it.

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