I've also noticed this flicker happening on Gold very occasionally when watching live and when this last happened , by rewinding and pausing at the point , i noticed a completely different programe being broadcast for a couple of frames duration. Exactly the same thing happened when watching GOLD on my parents Virgin Media connection last week, so is not just specific for Now TV.
The white box is there to clearly identify video streams originating from the app and website, to prevent their unlawful use. Happens on many platforms especially football.
When this was on the screen Gold worked perfectly and since it vanished there is now a crackling in the background.
I too have noticed, on two Now TV boxes and two Android devices, that UKTV Gold has very crackly sound - all the time. Not now and again - but constant! The little white square in the bottom left hand corner has gone, and the aspect ratio, like others are saying, is fixed. But the channel is almost unwatchable due to crackling audio on all programmes and adverts.
Will we ever get Gold without a problem???!