sorry mate struggling to think what it could be. Does it only do it when it is in Drive or does it do it in Park and Reverse too?
Yesterday I drove down into deepest, darkest mid-Wales and it was hardly noticeable all day. This morning, dropping the wife off at work, it's quietly knocking again - took about 1/2 mile before it started.
Having got home again I sat with the car idling on the drive. It makes the noise in all gears, neutral and park although N & P are decidedly less frequent. Putting the transfer box into neutral doesn't affect it either. At tick-over the noise is a regular "clunk" but when driving it sounds more like a rattle and it's most pronounced at about 1250 rpm.
Don't know if any of that helps.