See this post for Crop Circle discussion.
For those that don't know, there are areas around the globe where about 2% of local residents report hearing a hum in the distance. It's never close or loud. It's most often described as the sound of an idling engine off in the distance just at the edge of perception, although some hear it a bit more clearly to the point of it being a bothersome distraction. There is the Taos Hum in NM, England's Bristol hum, the Auckland Hum in New Zealand, and the Kokomo hum in Indiana, to name a few. In some places it is more likely to be heard by young people, in some places old people. Sometimes it's heard by more women than men, other times it's heard by more men than women. Some people hear it better up higher like in a tower or building. Some people hear it more clearly when they are below ground. Some people hear it all the time, others only occasionally, and still others hear it at night and others during the day. Some people find ear plugs help, others find that they do nothing. No one has managed to record this hum, although they can reproduce it with equipment.
One thing is for sure, there seems to be no pattern to who hears it or when or even how. The only common thread is these people all live in the same areas. Obvious explanations, such as industrial equipment and radio towers, have been ruled out, and no natural source has been found. Mass hysteria has been put forth as an explanation, but considering the number of people who hear it is actually quite limited, this seems unlikely. Folie en famille also doesn't fit in all cases. In addition, the people who hear it are not likely to be prone to mental illness. Tinnitus and the similar true phenomena of people hearing radio frequencies have been suggested, but the people who suffer from those things hear different sounds, and not the hum. There is a muscle inside your ear called tensor tympani which many people can voluntarily tighten to produce a low humming sound extremely similar to the The hum; a lot of people interpret this as hearing the blood rushing through their ears. This muscle sometimes tightens on it's own or in response to pain and can even get stuck, offering a condition that probably explains at least some of the cases of hum.
The leading theory is that the hum actually has many sources, some undoubtedly natural although undiscovered, and some man made. Others have medical conditions or have fallen victim to suggestion and confirmation bias. In other words, it's a collection of different phenomena that have been grouped together. Since the hum has yet to be recorded, it's very existence is still in question. Ultimately, even if we apply all these explanations collectively, that still leaves many cases of the hum unexplained.
But since research has produced no testable evidence, the only thing we have is personal experience. As with anything unexplained for which we only have anecdotal evidence, it is likely that when the phenomena is better understood the mystery will diminish.