MRI scans produce images of the body by causing shifts in a very strong magnetic field and then measuring how the tissues react. The average MRI scanner tends to produce magnetic fields 60,000 times the strength of the earth’s natural magnetic field. Inside the scanner there are coils of metal wire, which create a magnetic field when electricity is run through them and causes them to vibrate.
Although the pulses are what create the image we want to see, they also cause the undesired vibrations, which is what ultimately results in the loudness of the MRI scans. The stronger MRI scanner and the stronger the magnetic field, the louder it will be.
Unfortunately, there is no way to get rid of the awful noise yet, but there are ways to reduce it, by wearing ear protection.
Check out the NY times for more information on the MRI noise.