The sound is caused by ‘gradient coils’ that are being switched on and off at kHz frequency, which is in the audible range for humans. The sound is caused by those coils vibrating due to the interaction of the magnetic field with the electric current in the coils: they’re non magnetic but they still feel the ‘Lorentz force’. As far as calibration, there is a pre-scan step (which is one reason why MRIs can take awhile) used to optimize the RF settings to each patient. Patients come in many shapes and sizes so the settings have to be tuned to get a good image every time. I’m actually not sure of how often they need to be serviced, but it seems like the manufacturers are here checking on the machines pretty often!