If you run a motor at double its rated voltage, then it will draw a _huge_ amount of magnetizing current.
The increased magnetizing current will cause huge bearing loads and will make the motor sound really bad. There are probably also magnetic effects that directly make noise as well. The bearing loads will quickly (but not instantly) lead to bearing failure.
The increased magnetizing current will cause the wind temperature to rise very quickly. If the winding temperature is allowed to get too high, the magnet wire insulation will fail. Again this is _not_ instant.
If you catch the problem quickly, then the motor will likely survive the experience.
Robertmorkel, you are asking about a different situation, a motor operated at half rated voltage. For a properly sized fan load, this may not be a problem. Fans do not require much starting torque and are nice steady loads. The biggest issue is to make sure that the current drawn by the motor is less than or equal to the rated full load current.
Note: the torque of the motor does not drop when you reduce the supply voltage. Rather the _peak available torque_ will drop. The actual output torque will depend upon the applied load and the (now modified) torque/speed curve of the motor.
-Jon