Sometimes the sound is constant, sometime it constantly changes in loudness at between one and four times per second.
Some people perceive it as being louder indoors than outside.
The largest number of reports come from the southwestern US, the Pacific Northwest, and southeastern states. Worldwide, the Hum has caused such problems in the UK and Sweden that hum-hearer support groups have formed there. There are hum-hearer reports from Italy and from Mexico. On the other hand, US hum-hearers have traveled to China and S. Korea, and report that no Hum exists in the locations they visited. I find it difficult to believe that the military is constantly in contact with is sub fleet and because of this I tend to discount the ELF system as a source.