In my experience, a gentle rocking (or rolling) of the bike at low speed is due to the spokes on one (or both) or the wheels being loose. You need to check the spoke tensions asap because it is indicative of movement which will, over time, cause the spoke locating holes in the rims to wear. If this happens (or has happened) you may find that you can get oversized spoke tensioners (used to be available, but these days ... ?). If not you will need a new rim and wheel rebuild.
Anyhow, check the tensions soon, because eventually the wheel may simply collapse which ain't nice
Hope this helps