I'm due in a few days, and for the last day or two I keep getting regular vibrations right down somewhere on my pelvic floor. They're very subtle (like a very faint 'buzzing'), and a regular length - around 2 seconds with a 2 second delay. I think it's pretty constant, but I have to be quiet / still in order to feel it.
I'm not concerned about this as the baby's movement etc is all very strong and regular. But I'm very curious! I think I read that babies "breathe" amniotic fluid - could it be this that I'm feeling? I can't imagine what else it could be 
This is definitely not hiccups as baby gets those, and they're really nothing like this - much stronger and more obvious.