Strange noise at 60mph

Hi.

I have a Defender 90, 1997, 300TDI with 128,000 miles. I have noticed at speeds of approx 60mph I hear a loud whining sound coming from the engine. I am pretty sure it isnt a wheel bearing and definetly think it is coming from under the hood. Could it be the turbo, or the alternator on its way out, or exhaust blowing?. I am not sure. If anyone has any ideas I would be most grateful for their help.

Many thanks.

Kris.

Could be a bearing of some kind, perhaps the alternator as you said, might also be the water pump - after a run, have a look around the front of the engine to see if there are any water leaks coming from it. If so, it's probably that that is making the noise.