Laboratory processing machines can add clicks and other noise to magnetic-striped film. Over a six-month period, a click at 20-frame intervals was heard on 16mm magnetic-striped film; this became serious when the intensity of the clicks increased. The source was finally found in an area of 15 mils on a roller in a film processing machine. Below the surface of a polypropylene roller, a microscopic particle of magnetized ferrous metal was detected by x-ray; by degaussing and remagnetizing the particle it was identified as magnetic. Once the cause of a click is known, a simple procedure can be used for routine detection of magnetic inteference.