Chucking?

I have annoying chucking when running at constant speed on fairly smooth roads. I have an Andersen hitch, shocks on trailer, MorRyde pin box, air bags on truck, 24% pin weight and I till haven't figured it out. Trailer is level within a few inches at worst case. Sometimes it is just the combination of truck/trailer and in my case it has been impossible to find so far.
Good luck!