Try removing the front prop. Then going for a drive with the centre diff lock engaged. If the noise has stopped it's something to with the front half of the drive train. If not reinstall the front prop and remove the rear and go for a drive. Etc...

Tax free is nice. I've just got my '85 one registered historic.

@tottot it is a 1978, but it came with the old NA 2.5 diesel. It is definitely a Frankenbtruck. Thats' why i am happy to bolt it together any way that works. It seems that you and @Grebby are telling me the front hubs should always be locked...

You can't have the front hubs unlocked and the centre diff unlocked. If you do then the centre diff will sent power to the front prop as it's the easiest to turn. The front prop turns the front diff, which will then send to both or either...