Knocking noise at speed.

My truck has started making a repetitive knocking noise when travelling at medium to high speeds. The lowest I've noticed it starting up was about 30mph, but once it starts up, it sometimes persists even to lower speed.

It doesn't seem to get worse when cornering or braking, and in fact, braking sometimes causes it to stop temporarily.

I stopped and looked around, checking all four wheel for anything loose or hanging, and couldn't find anything visible, paying special attention to the rear wheels, as it sounds like it's coming from behind me, though it's hard to tell if it's the drivers side, or something closer to the center.

The noise seems to match the pace of the wheel rotation, being more frequent as I go faster, and it stops completely as I slow down, though at what speed it stops varies. It is loud enough that I could easily hear it over the radio and wind noise at highway speeds with the windows up. It is a quite distinct knock or clunk sound, not a whirr or hum or any other more contiguous noise.