CNC router build from Adept robotic cartesian slides.

[Frank S]

looking good. But I can't help wondering if all of those large hollow sq tubes that make up the frame will become resonating chambers. I am pretty sure I can already hear a machine that will deafen everyone around.
Foam filling may help if the time comes but for sure capped ends and oil soaked fine sand packed into all cavities will kill of many decibels plus effectively turn the frame into a dead solid block which will serve to cancel out much of the possible harmonic or possibly ultrasonic resonance if it occurs.