Noisy Disco - advice required

My series 2 (2001) Discovery has a significant increase in noise between 55 and 60 mph. This sounds as though it's coming from beneath the front seats or to the front of them - hard to say exactly. As I get beyond 60 it starts to quieten down again.
I have a suspicion it could be prop-shaft related (maybe the UJ?) or transmission. Any advice as to what may be the root cause would be appreciated.
Cheers