Driving the cos on country lane, changed from 4th to 3rd only doing about 50mph. The car made this terrible clunking rattling noise like something had fell off!!!

first thoughts were the gearbox or the diff. when the clutch is raised while the car is in neutral the rattling starts again and the car wants to stall, its as if the clutch is trying to engage. The AA geezer said it wasnt a drive shaft it was something internal instead. i hope it isnt the box. the car will still engage all of the gears which makes me think it isnt the box.

PLEASE HELP

martin