External Speaker Humming sound after PC shutdown

[JABLONe]

I have the brand new HP Envy All-in-1 PC 34-c1000. Excellent PC with one serious problem - the 3.5mm audio output jack. Works fine with external powered speakers (I have tried several from Bose, Sonos, audioengine, Edifier Luna) until I shutdown the computer, then I hear a loud humming sound. The humming persists even with the PC unplugged from power. The sound stops only when I power the speaker off which is not an option. Tried several outlets and different outlet combinations, different 3.5mm cables to no avail. Interestingly, with same setup on a previous 2019 Mac Pro, no hiccups. Very very annoying. Does anyone have and solved the same problem? Tired of all the suggestions so I'd like to hear from someone who really knows. Just freaking angry paying over 3k for a PC and can get decent audio output. 

[Prométhée · Accepted solution]

hi

so I know, it may have nothing to do, but it's worth checking, I've already encountered other similar problems
I don't have the exact model of your machine, but it seems that only one motherboard is possible
SO :

https://support.hp.com/ee-en/product/hp-envy-34-inch-all-in-one-desktop-pc-34-c1000/2101054301

Motherboard Specifications, Angel

HP ENVY All-in-One 34-c1999nz PC Product Specifications

it is mentioned, 1 Headphone/microphone combo port
so no other audio port it seems
it is likely that a "standard" connector whether it is a simple helmet, or speakers, will not be suitable, since it will require a connector for headphones, and microphone
see here

https://support.hp.com/ee-en/document/ish_4315149-4208232-16

Connectors on the computer marked with a headset-only icon support output sound only and work with either a standard three-pin headphone or a four-pin headphone for sound. 

I'm not sure, but it may be possible to use an adapter to get the right plug!

Connectors on the computer marked with a headset-with-a-microphone icon

support sound and a microphone when used with a headset with a four-pin plug and a microphone.

[JABLONe]

Thanks mate, I looked closely and mine is a headset (mic and headphone) combo. So I needed a TRRS 4-pin adapter to separate the audio output. Saved me a lot of trouble. 
Thanks