Thanks for your speedy relies guys. I checked the bolt for the distributor, seems ok. Exhaust leak make some sense, it is probably better described a growly popping/ spitting sound. It sounds similar to the leak this gentleman was havng at idle, but only occurs at higher, loaded RPMs, so with a higher frequency.
https://www.youtube.com/watch?v=44gUJGynnw0
There is no visible soot around any of the ports with the manifold on, but it stands to reason it could be a very small leak since it only shows the symptoms under conditions of maximum pressure, or perhaps between the muffler and manifold?
I could get a little popping by opening the throttle at idle but it doesn't sound very pronounced in the video, I will try and make one driving.
https://www.youtube.com/watch?v=taIsMF3UlCA