Moved my bed
I keep forgetting to mention that the one thing I have done prior to all these new vibration experiences -- three or four separate episodes since the beginning of the year -- was to move my bed.
As I've mentioned before, my neighbors here are very loud, so over the course of a few nights I found an area that seemed to be more quiet at night than the place I was sleeping. I moved my fake-futon over to that area, and I've been sleeping there every night since. I did this a few weeks ago, just after Christmas.
In the new area it is quieter, and the futon is at a ninety degree angle relative to the previous position. I wouldn't have thought to mention this, but in the early days Robert Monroe was very scientific in his approach to out of body experiences (OBEs), and he mentioned things like the direction in which he was laying.
I just remembered that I have a cheap compass, so I just looked, and I've been sleeping with my head pointing to the west, perhaps just slightly north of west.
I can't think of anything else that has changed here, other than the weather, which has been around 20-30 degrees Fahrenheit for the last few weeks, until today's warm-up, where it may have gotten to 40 or 45 degrees.