| Hmmmm.... Difficult to say without hearing it.
First thing I would say is, does the noise vary with engine speed or road speed? Does it rise in pitch when you go faster or does it stay the same droning note? Does it get louder?
Is it a constant 'wine glass' type noise or is it intermittent?
The next thing to ask, is it the engine or something else? One of the cycle parts rubbing perhaps? Since you can't hear anything when revving the engine at a standstill it might not be the engine. Try pulling the clutch in and freewheeling for a bit with the engine at idle. If the noise is still there it's more likely something touching somewhere than engine related.
If the noise stops when you pull the clutch and freewheel, but continues when the engine is under load it could be your clutch (at 17000 miles that's gotta be near it's life expectancy), having said that the clutch should only make any type of 'wine glass' noise if it's slipping and you would feel that when the engine revs rose without any extra drive. But, if it only happens under load, could be something like a gearbox output bearing.
Somebody with more of a clue might come along in a minute, but that's what I would be checking.  | 
Jools
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