The first thing I woudl try is to use render region instead of crop, I never liked crop too much, I'm not sure but maybe there is a camera related problem with crop because your seams seem to be related with the pictures not matching perfectly due to a different perspective, I may be wrong, but maybe crop is affecting that.
I think if you a render region, as long as you render the pieces up the the same sampling level, they should match perfectly, or near perfectly, I'm not sure if there can be any problem with the noise pattern, but it's a different problem than the one you show here.
Hope what I said makes sense :)
Cheers.