After much effort to find a hiss in my speaker systems I found when I removed the 2 RCA (1lft 1rt) cables which ran through a wall in a small plastic tube with 3 HDMI cables the hiss went away.
Is running RCA and HDMI close together an issue?
HDMI BJC bluejeans cable
RCA LC-1 bluejeans cable
Probably not for the HDMI cables. The RCA cables, since it is analog, could have noise introduced if they were running parallel. What were you using the RCA cables for? (just confirming line level analog audio)
Since you didn't get hum, I don't think it could be a ground issue. I'm guessing that when you ran the RCAs outside of the wall, that there was no hiss?