That's great news. Gerry was right on the money. I'm not running dampers on my X or Y, but like Ger said, every machine is different. Kinda weird that it just showed up after four years of carving though. I'd check all of your nuts and bolts for good measure too. Wood shrinks as it dries. I found some of the big bolts on the ends of my gantry plates were really loose after a year. Any vibration in the machine can cause the motors to resonate at certain RPM's.
I've updated my build thread with photos of my X gantry bearing mounts that you requested.