I have what sounds like a high tone resonance, frequency matches the engine rpm ,only under 2000rpm. I only noticed it after using earplugs to cut out wind noise ,I think the wind was drowning it out .
The sound seems to come from the front of the engine , dosnae sound like a cam chain ,is there anything else in there that could be causing it ?
Exhaust pipes have been known to rattle against the front engine mount bolt etc as they run quite close to several other bits of metal.
Give me a shout next time your going oot for a dirl ,I could ride over and let you have a listen . Could well be exhaust , I fitted stainless pipes off the later EV and they are bigger o/d ,I had to machine oot the clamps .
couple of hours on , re fitted the pipes ,lower sidestand bolts were fouling the pipe ,but that wasnae the cause ,all got around 4mm clearance and still the noise persists ,except now I have no idea were is is coming from ,it just seems to generally radiate from the lump under the seat . Not going out to try it because of the snow .
A dinae ken whit yae mean by a high tone resonance but ma Cali was makein a gey noisy ringing sound. It turned oot tae be the sidestand springs rattlin aff the exhaust. Ah sorted it wae a strip a rubber cut fae an auld inner tube, ah wrapped it aroon the springs tae damp it doon 
And again…in English pleez… 
…just jokin…I love the patter ! …
It’s nae a ringin soond , mair a woo,bit with a kinda warble tae it ,ken.