Failed IRD

Could do with some pointers, looked at a TD4 auto for another member that had a low knocking sound when moving, suspected a VCU bearing. So today went to drop props off to see to be told the noise was much louder. Dropped props off and went for a spin, there is now a loud knocking sound when both in drive and free wheeling in neutral. So front end in air and put into drive, when either front wheel has its rotation stopped there is no change to noise, so rules out bearings or drive shaft issues I am thinking. The noise seems to come from centre of car so has to be a IRD issue (main pinion ?) or in gear box as its a auto and I have no knowledge of problem with them, can it be something in the box.
Any pointers would be welcome

Centre diff mounting ???

Thank for reply, its a internal issue in either IRD which I am nearly sure it is, or gearbox, its just my knowledge of the auto box is nil, so just wondering if there are any known faults that the auto box that could cause the loud knocking.

Yes sorry with front end in air and no props attached that rules out diff. somebody will be along soon with a lot more knowledge than me so hold tight.

Grayo

engine / gearbox mounts ??

Sump guard?

probably a tooth off within the ird unit, suggest you have the plug out to see what is on the magnet, be ready with a piece of rag to block hole, inspect plug and replace.