Thanks guys I think you're onto something. StarCruiser you're right - it does decelerate the truck slightly and vibrates the steering, the whole truck really. Hard to pinpoint the location of the noise but the more it happens I'm fairly certain it's a front wheel or diff area. Like Rob I thought mine happened mostly when coasting and is definitely triggered when either of the front wheels hits a bump. This morning was probably the worst it's been; it did it for a good 30 seconds or so but unlike Rob's it wouldn't go away when I throttled on, or braked. However the 30 second grind fest seems to have triggered the ABS light, which is new.
Will update with what we find next.
So it took a while to get the cruiser booked in and finally sorted but you guys were on the money; stub axle brass bushes and stub axle washers have been replaced - also had swivel hubs rebuilt and back lash reset on the front diff while it was in. No more noise.