It's called a ground loop. No, not the taildragger sort. A ground loop in an electrical system occurs when the ground side of some part of the circuit is compromised (loose, dirty, corroded connection) and the ground current tries to find an easier path. It "loops" through something else. For alternator grounding, the path is usually via a cable that runs from the alternator to the firewall, and from there to the battery through the airframe. Aluminum airframes tend to oxidize and resistance builds wherever a terminal is grounded to it, so the current will trickle through other stuff and find its way, for instance, into the headphone jacks where they're grounded to the panel and through the intercom circuitry to internal ground points. It only takes a tiny bit of that rippley alternator current to cause that whine. Headset jacks should be insulated from the panel with special insulator washers and a ground wire run from it directly to the avionics ground to remove that path.
Fixing strobe noise is harder. One has to get under the panel, with the master on and the strobe running, and while wearing a headset, and start wiggling grounds and tightening grounds screws and so on. Sometimes fooling with a jumper wire from ground points to the radio stack can find a bad ground. I once did this with a particularly stubborn alternator whine by taking the cowl off, alternator belt off, and pushing a tablesaw up close to the engine and running the alternator with belt off the saw's motor. Saw blade disconnected, of course, then getting under the panel and so on. Found that the intercom box, a portable affair, had been screwed to the 172's metal pedestal structure above the fuel selector, and the metal case of the intercom was grounding the headset jacks in it. I pulled the screws out and fastened it with Velcro instead. Whine was mostly gone. I also found that a jumper between the upper and lower sections of the firewall cleaned up the rest of it. The rivet line between those sections was oxidizing and forcing the ground current into various undesirable places. I ran a light ground cable between those sections. The alternator was grounded to the upper section, the battery was grounded to the lower section. Easy to see that any resistance between those will deflect the current all over the place in the airplane.
Old airplanes are fun.
