MRI scanners map your insides by measuring how your tissues respond to changes in a powerful magnetic field. This magnetic field is created by running a high-voltage electrical current through coils of wire.
To produce shifts in the magnetic field, the electric current comes in pulses which oppose the field. This causes the coils to contract and expand ever so slightly, resulting in a rapid knocking or hammering noise.
Depending on the strength of the magnetic field, this noise can be as loud as 120 decibels – which is equivalent to a jet engine!