When ever I put my car into reverse(while the engine is still cold)and I step on the brakes. I can feel the brake pedal vibrating and I can hear a vibration that kind of sounds like it's coming from the engine bay. I can feel the car slightly vibrate when I step on the gas hard in reverse, while I can also hear it making a vibrating noise. It wasn't like this until my dad took it to Bridgestone to get the stubs for my wheels replaced and replacing my brake pads as well. The weird thing is...it only does it in reverse....and when the engine warms up, it stops vibrating in reverse when I step on the brakes, but still does it when I step on the gas pretty hard(barely)

Please help

 

So if you have been driving the car for a bit and it's warm it does vibrate?
It also just started this since you replaced the brake pads?
I would guess it's the pads. See when your driving the car the pads get worn to the rotors. Often this would wear slightly differently in Reverse. Is the car moving slightly when you feel this vibration? I wouldn't worry about it too much.