Vibrations in lower area

I'm 28 weeks, and I've been feeling a kind of vibrating feeling very low down in my pelvic area, it lasts about half a second and I get a few of them, kind of like when a phone vibrates.
Is this normal movement? I have an anterior placenta so I don't think my movements feel all that normal and frequent anyway, but the baby kicking is more of a popping or belly ripples, not vibrating though - that seems weird.