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.