This is the location of your root chakra, lower back along the spine, above the anus. Vibration is a very common thing as it begins to open and activate. Nothing to worry about but you should learn more about it. Mine often vibrates when I'm sitting or lying in bed and it feels like something is shaking the bed or walking nearby.
I have/Had spinal stenosis @ L3-S1 and did not have that symptom. The vibration went on in my lower legs. I could just sit there and watch my muscles twitch away. On 11/13/07 I had the X-stop placement procedure on L3-5 and my sciatica is gone (so far) and the muscle twitching. I do have alot of numbness and tingling of my right foot and leg, but since it's only two days post op, I'm not too concerned.
For those of you who have spinal stenosis, I recommend you look into this procedure. It is very new to the U.S., but has been done in Europe for about 10 yrs, I think. It's the placement of titanium spacers inbetween the vertebrae, allowing the nerves to be released from the pinching. The great thing about this procedure is that it is completely reversible if it doesn't work.
Another surgeon recommended fusion, but I wasn't comfortable with going with that-as a former ortho nurse I have seen too many fusions gone bad.
Back to you. Look into "Cauda Equina syndrome." I'm not sure, but the rectal feeling may be part of that....good luck