Thunking noise from drivetrain

The other day I had my window open and coming up to a stop sign I could hear and feel a thunking under my feet. I have no idea what it could be but it is only noticeable below 10 mph. At speed there is nothing. I do not have any rocks in my tires and everything is stock. Any ideas what this could be so when I do get it on the lift I can check those things first. It is a 2005 le crew cab with 70000 on it. Thanks in advance.