If you meant "Why is there a vibration, or vibration like sound when I'm driving" it is because either the drive-shaft is dented or one of the balancing weights is removed. Or basically it is not balanced, therefore vibration occurs.
u-joints....cheap fix....
it may be the drive shaft u-joints if its rear wheel drive
Check your universal joints!
drive shaft carrier bearing needs to be replaced
The chances are the shaft in question is most likely made of aluminium. The "tube"of the shaft will carry "noise" of the drive train. The covering is there to help limit the noise and vibration. It is more for dampening noise than a structural covering.
Typical U-Joint noise. Makes noise in reverse but not in drive. Check both front and rear U-joints of the drive shaft. If one is bad replace both. You do not want to go through R&R of the drive shaft twice.
Possible failed u-joint.
Well you can't actually accellerate with a broken drive but however if the drive shaft breaks while you are driving your vehicle will keep moving providing you don't press the brakes
Well the drive shaft. Transmission. is messed up!
Need to check the rear u-joint in the drive shaft.
it might be your drive shaft or transmission going out.
Belt Chain Drive Shaft ( Employs bevel gears ) Gear drive