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I'm with the votes for the noise being the straight-cut primary gears.
But all EX250s have these straight-cut gears, so why does this particular bike make more gear-whine noise than most others?
Two possible theories:
1. It could be that it's related to an oil flow issue to that area of the engine, somehow this particular EX250 engine is leaving these gears slightly drier than normal (less oil clinging to them or not being pumped to them or not being properly drawn up between them). Oil clinging to gears definitely quiets/mutes them a bit.
Or,
2. At some point in this EX250's history the engine oil ran low or got too old and the straight-cut gears got abnormally worn and thus developed larger-than-spec clearances, causing them to make more noise than normal.
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