Thanks Dorsal for chiming in on this one. I had assumed he had the external alternator. I didn't even think about the internal variety which is not really an alternator, it's a dynamo.

In any case the hunt is on for the noise.

Bryan, if you have the internal generator you can start by checking the output voltage with a simple volt meter. The Ducati regulator should hold the output at 14 volts DC. If the voltage is either above or below by more than +2 to -1 then the regulator is suspect. It is a less expensive place to start if you are going to shotgun the fix and it's plug and play.

Failures of the dynamo rectifier/ regulator, be it Ducati, Deere or Kubota is pretty common. I think they are all made by Denso with different labels pasted on them. but they get hot by design and can fry themselves. The intermittent characteristic of the noise leads me to think it is heat related.

The tach jumping up and down when you get the noise indicates a high frequency AC current is getting into the system. The Rotax uses a pickup coil for the tach. The tach sensor measures pulses from crankshaft rotation. If a sudden rush of high frequency AC current gets into the system from a bad regulator, the tach thinks the engine has gone wild and pegs. At the same time the audio system is telling you it doesn't like the AC current and it lets out a squeal. That stray AC current is not good for your radios, transponder intercom, etc.

If you have a friend with an oscilloscope it will be worth your while to buy him lunch. A scope can spot a failing rectifier/regulator where a simple voltmeter will not. Have your friend test the electrical system with the engine running and the scope set to AC. The DC bus should ripple a small amount but the voltage should not vary more than 5 milivolts. If it does, your rectifier/regulator is bad. The scope will also show each coil in the dynamo. If one of those is bad it will show a repeating pattern like blip, blip, thud, blip, blip, thud.

Failing to find a local guru with a scope, I encourage you to take the aircraft to an avionics shop. Even at $100 and hour it's less than replacing any one component in the blind. I'm also concerned that the spikes you are getting will damage other components and that could get real expensive very quickly.

Good Luck
JP