I had heard something about this in passing recently, but now I am very interested in it as more and more people are reporting the strange rumbling that they are hearing. It can last as much as 45 minutes.
Here is the map on people's reporting. Scroll over the pushpins or balloons to see how recent they are.
This quick video is an example of the noise-It's an older video but it's one of the few examples of the actual noise that people have caught. It's from Florida as that is one of the main areas where people are STILL hearing it.