That could be the source of the noise but probably not. Fluids tend to dampen out any vibration that a small part would make.Whatever is causing the noise, it sounds like something a piece of thin sheet metal would make if it was lightly vibrating against something. With that high of a pitch it isn't likely to be something large like a mud guard although the brackets on the fuel tank are small enough to buzz that fast. Maybe something in the horn?I don't think it's anything coming from inside the engine. Usually engine buzzes are directly related to the engine speed and change as the engine speed changes.As for the metal ring, if the filler neck on the tank is secure it might be something that was a part of the fuel pump. Maybe something that holds the fuel filter in place?Call up your dealer and tell him or his mechanic what you've found.They should know if it is something that needs to be repaired.