This problem affects many users and today I found the cause.
I dismantled my DCS-6020L; take the rubber feet off and undo the screws etc. etc.
The problem is the microphone is open circuit.
Looking at the wires, during manufacture they have been crimped hard at both ends.
I assume that one of the wires finally broke!
Removing the microphone, cable and plug - the unit is totally quiet.
So the hum is not coming from the amplifier; it comes from the microphone/cable/plug assembly.
If D-Link provided this assembly as a spare part (probably costing a couple of dollars) camera s with this problem could be fixed.
Also, if they told their supplier about excessively crimping the wires, it would also fix the problem in future.
Any idead D-Link?