Having my air on makes my car "pulsate"

[Sean Dodson]

Hello, new to this forum and hoping someone can help me out. When my car is on with the air off it runs just fine no weird sounds or anything. But as soon as i turn on the air, it could be with or without the A/C on and it could be hot or cool outside, the car makes a pulsing sort of grinding sound. I can feel it in the brake peddle, almost as if the car is breathing. I've tried turning the temp up (keep it at 73) and tried turning it down, this does not change the problem. I have a 2012 lacrosse premium V6.