R380 gearbox rattle under load

Been searching the internet on this one but not found anything like it yet. When my 110 is under load (climbing a hill or heavy acceleration) there is a rattle/grating noise coming from around the gearbox. It seems to also occurs if something causes the truck to drop into its suspension, by even a little, when travelling above 50mph; say a depression in the road. In the former scenario it doesn't matter which gear I'm in, just that the engine is under load. In the latter I'm in 5th gear.
When it happens I can feel additional vibration on the tunnel walls.
I've had a look underneath to see if I could see any loose linkages, heat shields, but nothing obvious.
Has anyone else experienced this and/or have any suggestions as to the cause/fix?

For reference on the state of the gearbox: the synchro into 2nd is a bit weak, 3rd is very slightly ify but OK. Otherwise no known issues.

Thanks.

how close is exhaust to the box

Just had a quick look. There is plenty of clearance between the pipe the the box, but there is a chassis mounted bracket about 1" from a section of pipe near the xfer box. Will have a look in day light and see if there is evidence of the two coming into contact.