The 3D vibration and 3D magnetostriction of a model transformer core made up of grain-oriented electrical steel sheets were studied by using laser Doppler vibrometry, an accelerometer and strain gauges. The following results were obtained. The magnetostriction of the transformer core at the T-joint and the corner area including the joint part displayed several times larger amplitude than that of a single sheet under sinusoidal excitation. The out-of-plane vibration of the transformer core showed a much larger amplitude than the in-plane vibration. It is considered that the out-of-plane vibration was the main cause of acoustic noise. The amplitude of in-plane vibration of the core can be explained for the magnetostriction amplitude in the in-plane direction. On the other hand, the out-of-plane vibration was tens of times larger than the vibration calculated from the magnetostriction amplitude in the out-of-plane direction. Furthermore, the same result, that out-of-plane vibration is larger than in-plane vibration, was also obtained by measuring a practical transformer core, and the frequency spectrum of acoustic noise presented a similar frequency spectrum to that of out-of-plane vibration.