I had the same issue with my car only it showed up at higher speeds (80 - 110). I had my mechanic check the driveline and all components and nothing was found.
I recently changed my wheels and tires and guess what......that's exactly where the problem was. If your due for some new tires take a look at the Hunter 9700 with ridematch when balancing your tires. It's expensive but the ridie is as smooth as butter.
If your tires are fine maybe you should look at rebalancing them with this machine. It will also tell you if you have any serious problems with any of your wheels or tires. Worth checking into.