There's a nerve thing some people get (you can also find posts from women on this site talking about feeling like they have a buzzing cellphone in their jeans pocket) and on this site there is speculation that it might be either a pinched nerve in the area, or even something in the lower back pinching a nerve. Try yoga for the lower back, and stretches, and if you do something such as bike riding that might put pressure on a nerve in that area, see if stopping it for a while also stops the vibration. Also, ask your doctor for a referral to a neurologist. From the number of posts about this issue, it won't be the first time a nerve specialist is asked about this problem.