The word "brontide" comes from the Ancient Greek word Brontḗ (βροντή), meaning "thunder," combined with the Greek suffix -ide.  It refers to a low, muffled, distant rumbling sound, similar to thunder but occurring without a visible storm, often associated with seismic activity or other mysterious phenomena.  It describes the sound like distant thunder heard in certain regions, especially coastal or lake areas, that are thought to be caused by weak earth tremors.  It is sometimes described as "thunder on a clear day."  The term can be found in scientific and nature-focused communities. 

According to the Oxford English Dictionary (OED), the word brontide is "not recorded in historical use."  The word appears to be a recent creation that entered English around the Year 2000. This is the key distinction to make, as some sources outside of the OED might cite earlier usage that the OED does not recognize.  The OED does, however, record the related but distinct term Brontḗism, meaning the style or manner of the Brontḗ sisters' writings.  That term dates back to 1887. 

Examples of "brontide" used in a sentence: 
*"What's a brontide?" she said, keep him from bolting after hearing the thunderous sound."  
*"Hiking along the coast, the campers were puzzled by the faint brontide they heard from the direction of the sea."  
*"On the clear day, a low brontide rolled across the valley, a sound that often precedes seismic activity in the region." 

Just the facts:

BRONTIDE Definition & Meaning - Merriam-Webster

Ok, guys, fess up.  How many of you are planning to use brontide in your next story?  😉

A new word for me but v apt as this morning we were driving south on the “Autoroute de soleil” towards Avignon and could just hear the distant sound of thunder through the torrential rain. And now I know I have the relevant noun to show off with! Thank you, @Myr