When I first built my machine, using proximity switches, I had similar problems and immediately worried about noise issues. Turned out to be a completely different issue - lack of hysteresis in the switches. All the reading I had done suggested that there was significant hysteresis - having approached its trigger, the switch would operate but it would then need to be moved a measurable distance away before releasing. Definitely not the case with these cheap Chinese switches - hair-trigger stuff around the switching point. So, Z axis homes, Mach3 sets it back from homing to limit switch mode, and Y starts moving. The vibration was enough to trigger the Z (now limit) switch and everything stopped. Sometimes homing would work and the machine would trip when I started moving away from home position at the start of a cut. The fix was simple - the Mach3 setting that moves the axis just off the home position once homing is complete.

I'm just glad that I didn't need to go hunting around with a 'scope, which I suspect throws up more worrying measurements than you really want to deal with!