01 Crown Victoria suspension noise

When driving down the road recently, I began hearing a slight popping noise. Over the next few weeks, it began getting louder and occuring more often. I dont have to hit a bump to hear it, just driving on a smooth flat surface will make it begin. I lifted the rear and spun the tires to check the axle, and everything is ok there. Checked all of my suspension bolts and that was ok too. Not sure what else it could be so any help would be greatly appreciated.