That's exactly what I had. It was (in my case) a worn 5th/6th gear and I could hear the backlash in the gears. It was sorted under the warranty ie. new gearbox fitted. Funnily enough I had covered about 20,000mls.
Is yours covered by a warrant? I was told that the gearboxes are known for it, type 32 or 34 cant remember.
My wife says that I never listen or something like that !
2.2 diesel Diamond 63. Swift Challenger 580