If by "lean" you mean that the plane of the wheel is shifting so that the top of the wheel becomes closer to one fork than the other, then it sounds like your wheel bearings are worn or loose. You can test this by holding the top of the wheel and trying to move it from side to side between the forks - if the bearings are in good condition, the wheel won't move in that direction.
If it is the bearings, the bike will seem to ride fine, which is consistent with your experience. However, the damage will be getting worse, which will eventually require complete replacement (which probably means a new wheel, in practical terms). You should get them fixed as soon as is convenient - it's not _urgent_ but leaving it longer will make it more likely that they need replacement rather than adjustment. You'll probably need to get a bike shop to do the work, as it's probably not worth your while to buy the appropriate tools and learn how to use them.