high pitched sound when driving

I have a high pitched sound while driving. It almost sounds like a turbine. I think its coming from the driveline or wheels. The sound picks up as I speed up and goes down as I slow. When I shift into neutral while moving noise continues. When stopped, the noise stops. Thoughs??

Sounds like it could be the transfer case. At long distance it's tough to know just what you're hearing, but a lucky guess on my part might help.

'99 Disco II
'95 R.R.C. Lwb (Gone...)
'76 Series III Hybrid 109
'70 Rover 3500S