Is it related to engine speed or road speed? If it's road speed it'll be something to do with the chain or final drive. Check the output bearing hasn't got excess play, that neither sprocket is loose, that the chain hasn't got excessive tight/loose play and the support roller, cush drive or cush drive bearing isn't shot.
And maybe consider wearing something a bit more than trainers and bare ankles when riding a bike where you suspect a transmission problem
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