The light probably has a Switch Mode Power Supply to convert the DC battery voltage to a different DC voltage or maybe AC voltage. It does that by rapidly switching the battery voltage on & off, then running this crude AC voltage through a transformer that changes the voltage. A SMPS may generate a lot of noise voltage. Take a battery power AM radio, tune it off-station near the low end of the dial and you can hear (maybe) this SMPS noise. Aluminum foil might work as a shield.