My house is semi detached,5 years old.
Over the last 9 months or so an annoying intermittent buzz has filled the house. It has slowly got worse and now wakens us/keeps us awake. We can't trace where its coming from. I get to a certain point in the driveway and can hear it outside too. Please help as its driving me insane!

Define semi-attached, please.

Sounds more like a relay or switch chatter. Do you have lights on a timer for outside? This is a common source of noise.

Shut off the main Breaker to kill all power to the home, see if that helps with stopping the noise.

Semi detached is a uk term for two house sharing a dividing wall. I have no lights outside, no timers of any sort.

Even with the power off the noise is still there

So , you have what we call in the USA is a duplex home, other wise two dwellings that share a common dividing partition.

Shutting off the power in your home leaves the power on in the adjacent home.

Have you checked with your neighbors to see if they hear the noise?

Is there something in their home causing the noise?

You need to do the work of checking the local area, getting other people to acknowledge the noise exists, and trying to locate the source of the noise.

We can keep guessing, but we cannot hear the noise over the internet, have no idea what the local area even looks like, so we cannot help with noise issues.