Gearbox noise

Hi I hope someone can help. I have a 2004 discovery 2 done 120,000 miles.
I have recently noticed an intermittent grinding noise when I’m in 5th gear and towing my caravan. I have not heard the noise when I’m not towing. The clutch was changed 18 months ago along with slave and master cylinder.
Thanks
John

I have had this when towing for long periods, I was told it's because I'm towing at low revs this wreaks the gear...to much strain. I have also had oil leak for same reason in hot weather. I changed gearbox both times for another used box. Both these occurred in Australia in hot weather. Hope this helps if it's the problem.