Noise/vibration when going down hill???

Hi all, I have a 2003 Defender td5, only just recently bought it and it has an annoying niggle.
When travelling down hill, as I back off the accelerator peddle to keep the same speed there is a grinding/rattle noise as well as slight vibration.
If I take my foot off completely, or accelerate down hill, the noise dissappears.
Anyone tell me what's going on?

Cheers!

Check your prop shaft uj's!

What am I looking for? Just excessive play in the joints?