That is switching noise, which is interference generated by all of the digital components in your PC.

Thanks for posted all of your components including the cables. Right away, the cables stand out as an issue. You are using an instrument/speaker cable, but what you really need is a balance audio interconnect cable. They look almost identical, but will have 3 conductors (TRS) on the 1/4" plug, instead of 2 (TS) that you have now. Some additional info if you are interested.

I prefer and recommend cables with XLR connectors. I use a pair of these cables between my digital audio interface and JBL LSR305's.

Potential issue #2 - make sure you set the input sensitivity switch correctly on the JBL monitors. It should be set to +4 dBu when connected to pro audio devices, such as your Scarlett. The -10dBV position is only for compatibility with consumer-level (E.g., RCA outputs from stereo, etc) audio, and you will amplify noise and get distortion if this is set incorrectly.