Are mysterious sounds, such as hums with no discernible origin, a new phenomena? Or just being reported more often with more widespread coverage in the internet age? Whatever the case may be, mysterious sounds with no discernible origins seem to be getting reported a lot lately.
One of the most common examples is just known as "the hum." It's a low frequency humming sound reported in various geographic locations that is not audible everyone. Here's a news article about a hum in Durham County in the UK. In another example, Canadians complain about a low pitched rumbling sound coming from Zug island on the US side of the Detroit river. But oddly enough, the people of the Detroit suburb don't hear it. Another example from the UK comes from Primrose Hill, where an
"unidentified 'deep resonating humming' coming from all directions at once" was reported.
But what about mysterious booms? A small Wisconsin town, for example, had reports of sounds described as rattling pipes, clanging metal, thunder, and firecrackers (but it was later decided that a 1.5 magnitude earthquake caused the sounds).
Another phenomena being reported are roaring sounds from the sky. They sound like aircraft, but apparently people don't see any aircraft. Also see another report here.
As odd (and random) as some of the sounds may be, there are probably simple explanations, such as the 1.5 magnitude earthquake in Wisconsin. But it may not always be clear that that simple explanation is.