are you getting the noise when you are changing from 4th to 6th or when going into 4th or 6th from a preceding gear
You will find known issue on here regarding gearbox issues if you do a search. Some owners have had there gearbox changed under warranty with a known crunching in 4th and 6th.
The issue is a known problem with the 6th gear synchro; this manifests as extreme difficulty engaging 6th gear from high revs in other gears (most notably 3rd or 4th but sometimes 5th). It does not affect downshifts (6->X) - normally anyhow.
If you have the above 6th gear issue, take it to a dealer (they should be aware of this problem) and they will road-test it and if they agree, replace the affected synchro (or even the entire box under warranty) but they do (did) have to send the gearbox away for inspection, so the car may be off the road for a while.