A low, rumbling noise people compare to an idling diesel engine, the hum might infuriate one person, while the man standing next to him hears nothing at all. It occurs the world over: there's the "Bristol Hum" in Great Britain, the "Bondi Hum" in Australia and the "Taos Hum" in New Mexico. One of the earliest reports of the hum came from Taos locals, who speculated that the sound was caused by magma moving beneath thin sections of the earth's crust or underground engines powering the nearby national science lab at Los Alamos. In 2003, two scientists studied radio-frequency emissions in Kokomo, Ind., in an attempt to explain that city's hum. They examined the town's 18 cell towers, the airport and the multiple broadcasting stations. Their conclusion: "If blood brain barrier opening by RF absorption plays a role in the acoustic transduction process, the radiation levels found here may be sufficient to excite auditory responses." Translation: You might not just be hearing things.