MRI Scanner Noises
The noise arises from the interaction of the gradient magnetic fields with the static magnetic field: the gradient coils are subject to a twisting force each time they are switched on and off. This results in a mechanical vibration in the "former" supporting the coils (it's a cylinder around which the coil is wound) and its mountings. They act as transducers and produce the loud sounds heard during an MRI scan. Those sounds are a loud humming and/or knocking.