As Richard said the LSD clutch can get noisy when the oil starts to break down and loses its "friction modifier" properties.
The drain plug is the very bottom bolt in the centre of the back cover and can be a bit of a bu**er to get out but a good mechanic with a long spanner should manage it. I changed the axle oil in my 2010 Roadster before I did the Austrian run earlier in the year including the Grossglockner pass etc and it made a big difference.
I tried the Redline that Richard mentioned and it was very good.
Can't be certain that it will cure your problem but worth a try.
