Loud vibration
 

Help Please

2007 CTS 3.6L

Help please. I've been experiencing a loud vibration during acceleration or while the engine is under load(wheels cranked all the way). At first it sounds like its coming from the engine bay, but when i roll the windows down it sounds like underneath. Also at low rpm-anywhere from idle to 2200 rpm. Vehicle must be in gear to cause sound. As i reach higher rpm it goes away. Tone of vibration changes with rpm.