They don't think it be like it is, but it do.
Buzzing noise from PC when I play a game.
Everyone I start a game I get a lot of interference noise from the speakers. What is causing this? They are LSR305s hooked into a Focusrite Scarlett Solo with 6ft RCA cables and the Scarlett is hooked into my PC via usb. Which USB slot I put it in doesn't matter.
It wasn't me!
Check to see if your system/mobo is properly grounded
Does it only happen when starting a game? Doesn't occur any other time, such as playing music or a movie on the PC?
I know monitors can be tricky sometimes (I have a set of Mackie CR4s, but only use them with my DJ equipment) but some monitors have good enough sound quality for other uses.
Question - Why the RCA to USB? Have you tried an RCA to 3.5mm adapter and connecting directly to the PC audio port (something like this: http://amzn.com/B000068O51 but obviously with a bit more length)?
Could be a bad cable somewhere along the path, or interference from another cable nearby (which grounding could fix, as suggested by AOD_Ghaylo).
If there's a way to eliminate the USB portion and go directly to the PC, it could help with troubleshooting.
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Change your audio output to 24 bit 48khz in your sound device properties. (This also happens when you have settings cranked up pretty high and your system is using a lot of power to run it)