loud noise following lightning: a loud rumbling noise caused by the rapid expansion of air suddenly heated by lightning 
2.  noise resembling thunder: a loud deep rumbling noise resembling thunder 
3.  threatening or vehement utterance: a manifestation of somebody’s anger in an explosion of strong words 

 As these 144,000 are anointed with Gods sign on their foreheads we continue to read >

that a voice from heaven , a voice like many waters and like the voice of loud thunder and then I heard the sound of harpist playing their harps . What a GLORIOUS  VISION THIS MUST HAVE BEEN TO SEE