Sump Pump turns on and off every 20 seconds... why? What can I do about it?

I think it is my sump pump anyway... the noise is driving my crazy. It's just a little hum, like the vibration when something mechanical turns on, that starts and stops every 20 secords or so. Its not the furnance, and the plumbing checks out, so I am thinking it might be the sump pump. It has been raining quite a lot, too. Any suggestions or thoughts? If not - who do I call? A plumber, electrician, or what? Thanks in advance!