xlr is a balanced connection, rca unbalanced, chances of you noticeing, slim to none
I am currently running my monitors thru 1/4 TRS > RCA right now and its getting mad interference from my PC. Probly due to grounding/routers/vid card activity but I just ordered xlr > RCA as I read they will work alot better due to being balanced so hopefully it fixes my probIn analog cables, it does make a difference. From my guitar/pro audio experience, 1/4 TRS has the possibility to absolutely suck. Never once had a problem with XLR. Don't know how that would compare to RCA though, never noticed a problem with RCA, especially in a home.
I have an onboard soundcard, internal soundcard AND an external sound card and I get noise from any three of the optionsI have rcas from my djm-707 mixer to my krks and i use a 1/8" stereo minijack to rca cable from my laptop with noise. Is your outlet grounded? how about trying turning off all your sound card channels in the mixer one at a time to see if one option is outputting your noise.