I am trying to setup a probe and my home switches on a Momus cnc machine Ive just finished building. I connected a wire to pin 15 and another one to 5v on my BOB. right away I start to hear an odd hum and when I touch the two wires together the machine goes crazy, grinding and trying to move. Its really rough sounding. I break the connection and it goes away. Ive tried active high and low and it still does it.
I tried connection a home switch to my X axis and I get a similar thing happening there too. So I did a little investigating into the problem. I found if I just connect a single wire to ANY input signal then I touch that wire to any metal on the machine it does this crazy grinding moving thing. It even does it through me, meaning if I touch the end of the wire the touch the machine with my other hand it does it just not as harsh.
This is driving me nuts, Ive checked all the common things like my gound on the power side of things, I used shielded cable throughout the machine too.
I do have some of my signal wires from the BOB running alongside some of the wire from my power supply to my motor drivers, but I thought since everything is shielded this wouldnt matter. Maybe it does?
Any suggestions??

I think the pin 15 should connect thru to ground, not 5v. Would have to review the BOB.
Signal wiring should be separated from power or stepper wiring even if shielded.
It sounds like you have grounding problems. Do you have a star ground? Are your shields connected there at the controller end only? Is your power supply grounded there also?