Reverse backup camera noise feedback

Hi all,

I have an Xtrons H/U (yes I know they are not great but it serves my needs). I have attached a reversing camera that switches automatically when i put the vehicle into reverse ... so far so good, however ...

If the external lights are NOT on, they're is a buzzing noise when i put the car into reverse << PROBLEM
If the external lights are on, no buzzing noise

FYI, The picture is perfect whether the lights are on or not!

Any ideas on how I can get rid of the noise??? Is it a voltage problem??

I have so far checked all connections and the earth is good (connected to chassis close to camera).

Thanks in advance.

-------------
Its not a stupid question if you don't already know the answer!