Loud rumbling noises, coming from Isaac?

I can't sleep, so I was outside watching the clouds about 10 minutes ago, and the neighbor came outside, along with my roommate, and we all heard the same noise.

I live over in Dunedin, north of Clearwater, FL. It's still going on outside right now, and we can't figure out WTF it is. There is minimal wind, and no rain. Deep cloud cover, but they're moving fast.

At first it was thought it be thunder, but that was ruled out. It actually woke the neighbor up, hence the reason she came outside. She thought it was thunder as well but couldn't figure out why it was lasting so long. It sounded like a tornado passing by, being such a deep, echoing sound. But there's zero visibility with the clouds so thick, and it being 1:00 a.m.

The weird part that is that the rumbling would start slow at first, then build up, get so loud you could feel your chest thumping, and suddenly stopped like someone flipped a light switch. It seems like it's timed, or in exact intervals. 3 -4 minutes on, 1 minute off. We stood outside listening through 5 cycles of this. Still no rain, but some light ground wind.

Checking radar and satellite now, and there's barely anything overhead. The area is under a tornado watch until 9:00 a.m., but there weren't any alerts of a touchdown. It seriously reminded me of those weird sky noises that were playing all over the internet a few months ago. No currently updated weather alerts for the area, but this is still happening, I can here it through the bedroom walls and window.

Any thoughts? If you're in the area, stand outside for a few minutes and listen. There are no birds, crickets, anything chirping right before it starts.

Need some input, thanks!