I am not sure what caused the noise but it does sound like its coming from the fan and I am sure the dealer has changed the unit because they deliver it to my door and took the old one back.
From what I can gather ( by talking to my dealer ), they are not sure what caused it as well.
The noise is immediate when I switch on appletv or ps 3 ( both connected using hdmi ), when just watching free to air tv the noise comes in gradually.
I will give my dealer another call today and see if there is a firmware coming up thats going to solve this problem.
Thanks all for the reply.