Sorry..... Been away doing "real life" stuff, like planning a new deck!
Well, since it is OK when the engine and alternator are not running (sitting in the garage during a waxing - I hope pulling the wax off left the skin smooth and shiny), then here's what to look for and try:
1. Put a voltmeter on the alternator output to see if it is overcharging. 13.0 - 14.2 is good at 3,000 rpm. If it's rising above 14.5 then I would consider a new/different alternator OR maybe get an auto electrical shop to install a new regulator in your current alternator. Your light bulbs will last longer, too.
2. If the alternator is more-or-less OK, then the radio may have an over-voltage protection circuit that opens the +12V if it sees something higher than it wants, like 14.4 volts at high RPMs. It acts kinda-like a circuit breaker that self-resets. If that is the case, a simple easy fix might be a radio-only voltage regulator like this:
https://www.amazon.com/Voltage...10-36V/dp/B06Y5JHZG2
3. If you still get a persistent alternator whine after installing a radio voltage regulator (or even a new alternator), you could try running an alternator noise suppressor at the radio like this:
https://www.crutchfield.com/p_...xess-AX-ANR1000.html
They're wicked cheap (considering that there's about 50 cents worth of parts in them) and usually work like magic but will NOT suppress ignition (spark) noise - just alternator and/or heater fan noise. You can run a noise filter AND a voltage regulator in series at the radio, no problem. The noise filter is 1/2 of a transformer (technically called an inductor) that smooths whatever signal is sees going through, thereby removing the noise spikes.
For spark noise you'll need resistor spark plugs or resistor (spiral-wound) spark plug wires - either one, but you don't need both. If you have an HEI ignition, stay away from resistor distributor rotors - the higher HEI voltage tends to roast them and they open up and go dead (ask me how I know). I have an old school ignition with lots of spark noise, just to annoy Porsches when I pull up next to them at a light.
If you live near a Best Buy or other place that installs auto audio stuff, drop by the installation bay and ask if they have a good, cheap radio noise filter. I used to put them on everything without even asking about the alternator. We'd buy them by the hundreds for about a buck apiece and had a drawer full. Best Buy might, too.
Here's a good article at Crutchfield on noise suppression:
Scroll down to "Radiated Noise" and then read "Engine Noise" right after that, too.
https://www.crutchfield.com/S-...tallation_guide.html
Good luck sleuthing..........
Oh, and make sure you have SOLID grounds for the battery AND the radio. We often overlook solid grounds on these cars but since we're dealing with fiberglass non-conducting bodies, strengthening the grounds is always a good thing.
