Freightliner century 2006

Maybe an air leak draining down your air bags and the truck creeping a little as it settles?

 

Mine was doing the same thing. Check for rust on the slip joints. There is some stupid plastic sleeve on it that traps water and rusts the shaft. This creates a bulge and the joint cant slide in and out smoothly.

 

Do you set both brakes if so trailer probably dumps suspension air and pushes on truck kinda feels like someone bumped into you

 

If you have an air leak and the airbags drop, then it changes the driveline angle.

I had that thump and bump for years, until I cut off that plastic and removed the rust. Also 06 Century.

Even if thats not it in your case, I would still remove that rust because it can apparently cause vibration

 

Air bags dropping. I have the same truck