My Desktop Computer gives humming sound

Hello Sir,
We thank you so much for the great work that you are doing for mankind.
We have a desktop we setup in the church temple. Anytime the Mixer is connected to the computer through XLR-Jack Audio cable it generates a humming sound.

When the same cable is connected to a laptop no humming sound. When the Desktop is placed closer to the Mixer with a shorter audio cable no humming sound.

we used an external sound card Onyx Mic Preamp but issue remains.

We did all that we could but to no avail.

Grateful if your outfit could help us fix it