Until you 'intro' yourself... you may not get lots of help...it is a custom here to make a nice intro w/ your general location IN THE INTRO TITLE.. the help we give is different depending where you are. AND you WILL get lots of nice greetz and some may be from members near you. IF you make a proper 'intro' confused? read lots of threads in the welcome/newcomer section.. Not doing that can be seen as being rude and a lack of respect.
ARE you 100% sure your RGBs have galvos and not steppers? They normally make a lot more noise than galvo engines.
some might say 'just crank the music up'.
Tim Walsh made a laser show module using the high pitched sounds that galvos make- it was quite interesting.
Since you have two (virtually) identical channels, you have the convenient option of swapping parts, cables, and signals between them to find the culprit. Try this and report back.