Whine can be caused by a bad ground somewhere. A "ground loop" is formed, and it has nothing to do with taildraggers. If, for instance, the alternator ground cable, between the alternator and firewall, becomes corroded or loose, the alternator's ground current finds other paths such as through engine controls, which carry it directly to the instrument panel, where it can cause noise by entering the headphone jacks, which should be insulated from the panel and a ground wire from them directly back to the radio stack. That alternator ground current can also make oil temperature gauges read high. It runs though the engine case into the temp sensor, and from there to the gauge and then to the bus. That little extra bit of current therefore pushes the sensor flow higher and makes the gauge read high.
Some alternators have capacitors (condensers) on the alternator case, and connected to the output stud or field stud, and in some cases there will be condensers on both. If those are loose or shot you get noise. If there's a filter on the firewall that the output cable goes to, that thing might be loose on the firewall, or corrosion has isolated it from the firewall.
Lots of things. I remember chasing a whine for a day or two. I finally rolled a table saw up to the front of the airplane and ran a V-belt from the saw's motor to the alternator pulley, and let the motor drive the alternator for some time while I probed around under the panel and along the firewall, all while wearing a headset. I found that the firewall was made of two sheets, and the riveted seam had oxidized and made a bit of resistance that shunted some of the alternator ground flow to some less desirable path. I made and mounted a short ground cable across the seam and got rid of some of the noise. The rest of it was coming through the portable intercom, which someone had helpfully mounted by running screws through its metal enclosure and into the pedestal structure. Ground current was getting into the system via the now-grounded headset jacks. I removed the screws and mounted the thing with Velcro and stopped the whine.
I have a small oscilloscope I occasionally used to see the alternator's waveform. A defective diode would be instantly visible. Never found one.
