Found this glitch today playing Demolition. When you're planting/defusing the bomb, if you spin your camera around while you hold X to plant/defuse, the enemy doesn't hear you open up the suitcase and push the numbers to plant or defuse. In other words, if you spin your camera around while you're holding X to plant or defuse, you're silent.

Steps:
1) Hold (X) to plant or defuse the bomb
2) As soon as you press and hold (X), push the right joystick all the way to the right or left and hold it until you're done planting or defusing the bomb
3) Win.

I even tested this out in private match with a few friends and no one can hear you plant or defuse if you spin your camera around while doing so.

Have fun guys!