Knock on the floor board

[Nice rig]

I'm starting to notice a knock feel on the floor when going straight at about 65-70mph. It seems to stop when I go around a long turn at the same speed, but then picks up when I straighten out. I still have original shocks installed and nothing else has been done to the suspension or any other component. Any ideas would be preshiated. I was able to do a good bit of driving yesterday and paid a little more attention to it. I can feel it as slow as 50mph and it does travel up to the steering column with a little bit of noise.