Help! Knocking while driving, not while turning
 

I have an 06 with no modifications. I was driving down the freeway at 65 mph and something very clearly fell off my car, then proceeded to bounce along underneath until gone. Immediately, I started hearing a loud fast knocking sound that would slow down as I slowed down. Pulled over to see nothing visibly wrong.

Drove home slowly and noticed the sound comes back after about 30mph. Knocking gets faster as I drive faster. It also seemed to only happen while driving straight. When I turn, it goes away. Come to a stop, it is gone.

I can't tell if I notice a problem with the steering or not. I feel paranoid about it, but I feel it is a tad bit soft and unstable. Could be my imagination.

I've gone underneath once and checked brake rotors, calipers, and various bolts and the end links. Can't find anything wrong. I can't really tell, but I feel the noise is coming from the center of the car, rather than a side, but I am not sure. I have nobody who can drive stick in my neighborhood to help me trouble shoot.

I'm getting back underneath in about an hour to try and figure it out again and remove the aero panels for a more thorough look. Anyone have an idea on where to look/what to look for?

May have left out details. Ask me questions and I'll answer.

I would check the ball joints. Have you had any work done, like the control arm recall?
One thing that commonly falls off older cars is the exhaust or cat converter heat shield.

The joints looked fine last I checked, but I'll look again. I had the rears replaced a couple years ago after they failed (Edit: Rear control Arms and something else that I forget). Fronts were never done.