Update: I think i've found out the reason for the strange sound and vibration... It's the image stabiliser at work. I've tested it by switching off the Steady Shot and the sound and vibration was gone. And if anyone out there who don't mind, could you please kindly test it out?
SSS should not have any strange noise or vibration during long exposure ! ...did you mount the camera on a tripod ?
When you switch on or off the SLT, you'll feel a very short vibration, that's the sensor shaking itself to get rid of any dust on it.