There are two seemingly similar functions that take a parameter kSystemSoundID_Vibrate:
1) AudioServicesPlayAlertSound(kSystemSoundID_Vibrate);
2) AudioServicesPlaySystemSound(kSystemSoundID_Vibrate);
Both the functions vibrate the iPhone. But when you use the first function on devices that don’t support vibration, it plays a beep sound. The second function on the other hand does nothing on unsupported devices. So if you are going to vibrate the device continuously, as a alert, common sense says, use function 2.
Don't forget to add:
#import <AudioToolbox/AudioServices.h>