What is the source of the loud noise of an MRI medical scanning machine? Is it due to some mechanical moving parts such as pumps, or is it an effect, similar to magnetostriction of superconducting electrical magnets?
Along with the main high field static magnet there are sweep coils that generate a changing magnetic field to align the atoms in a slice that moves accross your body - this is the section that the machine 'images' at a time.
Since the coils are rather powerful and the field changes direction abruptly at the ends it puts a large force on the mechanical mountings in the machine - it is this that makes the noise. There are designs which have pairs of coils producing sets of sweeps which balance the force on the coils and so reduce the noise - but this adds to the complexity and cost of the machine.