Well that's annoying

[Cricketbat70]

Unlike many on here, I don't have a decent desktop setup for music as its not my main way of listening to music. I just have a sony SRS-X33 Bluetooth speaker connected via 3.5mm cable to the audio out on my PC. Until recently I was using a 15year old tower PC. A couple of weeks ago I bought one of those ebay specials. Ex business mini PC, nice and tiny and just the job for me. Anyway yesterday I wanted to listen to a bit of music and boy the humming coming from my Sony speaker. My first thought, the audio out on this mini PC is no good. I never had an issue with the old PC. Today I tested my headphones, everything was great no humming, so I thought maybe the 3.5mm cable so I tried my son's Pure Voca speaker with the same cable, again no problem. So it would appear that although the cable has never been taken out of the sony speaker in all the years I've had it, the 3.5mm socket in the speaker has an issue. I could understand if the cable had frequently been plugged in and unplugged.
This is not the first Sony I've had an issue with concerning the 3.5 input. I am going to try another cable just to be sure but so annoying.

 

[Vincent Kars]

Sounds like a ground loop to me.
Try connecting the Sony using Bluetooth. If the hum disappears it is likely a ground loop.

 

[Cricketbat70]

Sounds like a ground loop to me.
Try connecting the Sony using Bluetooth. If the hum disappears it is likely a ground loop.

I am going to try with the Bluetooth. If everything is fine then a new USB cable or plug as that is the method of powering/ charging it.

 

[Gray]

I am going to try another cable just to be sure

Try a different line source into the 3.5mm socket.
Would be a coincidence if the socket is exhibiting a fault just when you've changed PC, but still.....

(Your little Sony looks to have a very similar form to my Kitsound Hive BT speaker - which can sound unfeasibly reasonable for what it is - rated at just 2.5W RMS per channel).