Hello,
I have making tests in Debian 10, Mint 19.2 and Ubuntu 18.04, these 3 OS all suffer from the same problem with the priose 3.5mm jack that whistles when headphones are plugged in and no sound is broadcast.
The problem is related to the Realtek ALC3266 sound card.
If you open the sound settings software, the whistle disappears, as soon as you close the whistle comes back.
If you open VLC or any other player, the whistle disappears, as soon as you close it the whistle comes back.
When there is no signal sent to the sound card the problem is present.
One way to solve this problem would be to continuously send a "blank" signal (just a signal without sound) to this sound card.
After tests, this laptop works perfectly under Debian if it is connected to a "Sharkoon Gaming DAC Pro S (USB-DAC)" sound card or on a "USB-3 type C to 3.5 mm jack adapter". But it does not work properly with the Realtek ALC3266 integrated sound card.
Yesterday after I made an installation of MX Linux 19 on my laptop that does not use "systemd" but "init", I noticed that I have no problems with my headphones connected jack 3.5 mm to the Realtek ALC3266 integrated sound card !!!! All work perfectly :) But I dont like realy this Linux distribution...
I hope that these additional indications will allow you to solve this bug which seems to exist since several years and has never been solved...
I hope that with a new look at this bug, that you finally will fix it !!!
Best regards.