Instead of hearing thunder claps, I heard just the rumble, and it was stretched out to about 15 seconds. This happened multiple times. A google search only leads me to recordings of thunderstorms. There's nothing like I've heard last night. One site says it could be the sound of distant thunder being refracted through the atmosphere, but this was incredibly loud.

Has anyone ever heard of this or was I just having a dream about being in my room unable to fall asleep?

But instead of it being the normal really loud POP of close lightning, it almost has the same volume but more of a train-locomotive rumble and seems to drag on? Is that what you're hearing? 

And when it has happened to me, I'm never startled by it. I'm never shocked out of sleep as if it was someone at my door. I sort of awake, acknowledge the rumble, then drift back off to sleep. I figure it has something to do with the brain lowering the senses while sleeping so that you don't wake easily. It would fall under the same reason that we don't usually snore or talk loud enough to wake ourselves up. So the sound of the thunder is slightly diminished. The elongated sound, I always assumed it had to do with the brain tending to process things in twilight like a dream, where one small thing gets repeated so a half-operational brain can process it. 

That very well describes what I'm experiencing. I felt really lucid throughout the ordeal so it just struck me as very odd and creepy. At the time, I figured such an unfamiliar sound could've been some secret vessel flying over in some sort of war scenario... then I realized that the US is hardly ever that exciting.