Intermittent noises ex MOD 110

Have an ex MOD 110 makes intermittent noises. Sounds like clonking of small metal pieces, then it gets faster and changes into a grinding noise.
Sometimes I can go days without it happening. Have tried seeing if it happens in different gears, neutral, hi or low diff, reverse braking etc. The noise happens regardless. The only thing I can say is that it does not happen when stationary even revving the engine, also when it starts grinding you do feel a pull on the engine. Has been going on for months now, on and off.

Wheel bearings?, propshaft u/j?, the possibilities are endless. Jack up each wheel in turn and check for play and get underneath and check the propshafts for play. Remember to chock the wheels when you jack it up if you're going to stick your head under it.