This photo is from a Samsung DV409AEW, but I assume they are quite likely the same. I also turned the drum (clockwise, which is the wrong way) and the same thing happened. The thud in my case was the spring that holds the tension pulley arm in place falling to the bottom of the dryer. If your situation is the same, spinning the drum caused the belt to come off of the idler pulley and the tension spring fell off. It seems to happen frequently based on web searches.
Again, assuming the DV409Axx is the same, you can get at it from behind. There should be 3 screws (2 on the top and 1 on the bottom) of the plate that holds the vent pipe output assembly. That assembly can then be removed. There will be a vent pipe extension that comes out with the assembly.
Once inside, see if the spring is off, and if so re-attach it per the photo below. It is counter-intuitive that the spring pulls the arm down, rather than to the left. Hopefully this will be your issue.
Putting the pipe extension and outlet assembly back in can take a bit of fiddling... but if you lay on the floor, you can actually shine a flashlight in the crack to see what you're doing.
Hope this helps!