Sounds very much like your alternator is failing to supply proper charge. However they would normally either fail completely, or be worse at tickover than high revs. Your combination of noise and failure to charge at high revs is more unusual. Perhaps something else is jamming at high revs, making the belt slip and failing to turn the alternator sufficiently.
Really you will need to look at all the pulleys being driven by the same belt as the alternator. To do that, you will have to remove the viscous fan, which requires a special tool or careful use of a cold chisel and hammer to jolt it loose. It's fixed using a left handed thread.
Once the fan is off, remove the belt by first lossening the tensioner, then turn each and every pull by hand, trying to feel whether anything is jammed or a bearing has failed.
If all the pulleys and the tensioner are ok and the belt looks in ok condition, I would replace the alternator next.
I would advise not driving it as it is because there is a good chance you will get stuck. Current: 2010 Range Rover L322
Vogue SE, TDV8 3.6, Bournville
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Now gone: 2002 Range Rover L322
Vogue, TD6, Giverney Green
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