Since the early 1990s a fraction of this town's residents have heard a low steady hum that the rest cannot hear at all. In 1993, under pressure from complaints, Congress commissioned a study by Sandia and Los Alamos; instruments in the homes of hearers recorded nothing matching the descriptions. The hum is described as a distant diesel engine, it keeps people from sleeping, and there is no shutting it out — earplugs do not help.