2003 Dodge Caravan knocking sound

Thanks! My Dodge Caravan is making a slight knocking sound when we travel slowly (5-15 mph) over somewhat uneven roads. The sound disappears when we start to pick up speed. There is no lknocking sound if the road is flat. It sounds as if it is coming from the front of the van, at the steering column. Not sure though. What do you think is wrong and is this something that needs to be repaired asap? I was planning on taking it to the dealer this coming weekend for them to take a look at it. I do not drive this vehicle, my wife does with my three young boys so I am very concerned. Can it wait that long? Or should I take time off from work to bring it in?

The ride is smooth and the van handles very well.

Thanks for your help!