Is there a chance your phone is sitting on the desk anywhere near the speaker or headphone cable? GSM/LTE phones can cause interference with the cables.
Does it get louder or quieter with the speaker volume or system volume? This can help determine if it's just background noise on the audio system or if it's something being generated by the computer. If the system volume affects it then it's being generated somewhere. If it stays consistent, or only the physical speaker volume affects it then it isn't system noise.
There's a chance it's just background noise on the sound circuit, most integrated chips aren't totally isolated and can be affected by fluctuations in power on the rest of the system.
If it isn't so loud that it disrupts the sound of anything you're watching or listening to, I would ignore it as that.