Marty, thank you for the suggestion. I'm guessing you mean the sealing rubber? It's not a plain rubber washer, it has a lip to fit over the flange of the cap. So I cut a thin rubber ring to fit
inside the seal, making it thicker, hoping that would hold the cap better and eliminate movement. Certainly, the cap felt more secure with that change.
Went out for a run - the resonance was quieter but still there. Further exploration with my finger - oh, do be quiet at the back - pressing on the lock stopped the sound but pressing on the middle of the cap did not. Conclusion - it's the lock that resonates.
Back in the garage, soon confirmed found that elements of the lock are all quite loose. It's a cheap and nasty lock. There followed a saga of trying different things. It all went a bit
Oooh errr! when I started to dismantle the lock and the return spring rapidly completed that task for me. Took 2 or 3 attempts before I learned how it all goes together and works. Various attempts to fit gasket paper shims inside the lock to take up clearance. Unfortunately, paper shims either didn't take up enough clearance or jammed the lock.
Was looking at the problem with Mrs when the same idea came to both of us. 'Would the lock from the old cap fit on the new cap?'
Only one way to find out:
That's the old lock fitted to the new cap. Only took a little bit of finessing with a file to make it fit. The new lock, dismantled, is on the tray. Note bung to prevent accidentally dropping small screws or WHY into the tank.
The lock from the original filler cap is much more robust than the Heiden-supplied one. And another test ride quickly confirmed that the annoying rattle, or is it a buzz, has gone.
Additional benefit - the Yamaha only needs two keys now. The same key fits the petrol cap and the seat lock. And the old lock definitely seems to hold the cap shut more firmly.
Onwards and upwards!