I just swapped alternators in the 10 to get rid of a slight whine and the new one does it also. So, I've tried this on 4 denso type alternators since. All 4 same thing, when you apply power to the regulator they make a little high pitch whine (F above middle C). They do this sitting still or running and I can hear it from 10 feet away in a quiet hangar. The noise that carries through the radios matches it in pitch and does not vary with RPM. I've been able to filter it out of the radios, but I can still hear it outside the plane. Anyone else notice this?