I do not suspect a problem with the 5. gear, because you said, the noise you hear is related to rpm and not to a specific gear. If, then it should be the clutch, which I do not believe in. If it was the clutch, you would have problems in lower gears too, e.g. from the 3rd on or so.
And, last, but not least: That would not explain a rpm-related noise from the engine that leads you to suspect the valves to make noise.
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Cuore L251 Bj 7/2003, Automatik:  Ausrangiert, leider!
Citroen C1 Automatik BJ 2011: 
Mofa: Dreirad auf Basis eines Amsler-Pony, Verbrauch Zweitaktgemisch: <3.5l/100km.
Das grosse Artensterben auf dieser Welt wird den Menschen erst bewusst werden, wenn schliesslich auch der Tiger im Tank ausstirbt.
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