tosh,

I had to replace my amal valve recently because it was "clicking" when you wiggled the wires that go to it.

Maybe at a certain rev, the vibration frequency is making internal contacts move, hence it clicking on/off very fast and making the noise you descrfibe.

Try to just switch the ignition on, don't start the engine, and wiggle the wires, if it clicks on/off, I suggest you replace it.

Also, do you have a turbo damper ? If it's not adjusted properly, it would make a "zing" type of noise at certain revs too.

Nothing concrete in the above, but I hope it helps all the same