I have this problem that when I know that the such ambient noise is synthesised in any way, then my brain simply rejects it as if I knew it is a placebo so it no longer works.

Similarly if I listed authentic recorded ambient noise, once I recognise a loop point, it becomes extremely annoying.

That's why I can't use tools like mynoise.net.

I had a couple of solid 8 hour recordings from a cafe or office, but after a couple of listens I kind of learned it is a recording and they no longer work. Basically rather than focusing on doing work etc. my brain is listening for faint phrases, whether I heard it before etc.

The workaround I found is that I just have a window open so I can hear street noises, but this is going to be troublesome during the winter.

I am planning to start making recordings so maybe if I have a solid month worth of ambient noise, I will be able to trick myself it is not a repeat if it takes a month for a full rotation.