when accelerating under power at about 2000rpm, I start to hear a rattling sound which stops when I take foot off accelerator

 

There are a few things it could be. If it happens under power but not on coast, you're looking at something rotational. It could be worn ring and pinion gears, pinion bearings, or u-joints. U-joints and cardan joint you should be able to push and pull to see if there is any play or movement, you can also grease the stuffing out of them and take a drive to see if the noise changes.

You can unbolt the driveshaft at the pinion flange and shake the flange to see if there is movement from the pinion bearings.

You can drain the oil in the differential and see if there is a lot of metal on the magnet to indicate wear of the ring and pinion.

Good luck!!