Hi Mr. S,
Weakening of the core muscles is definitely a common feature of MS, and yes, for a lot of people, this does seem to lead to back pain (I’m another one!)
A lot of prescription physio is actually based on Pilates, which aims to strengthen the core.
As an aside, I get the awful joint-clicking as well. I would certainly not be able to creep up on anyone, as they would hear the crunching and clicking a mile off.
This, as far as I know, is NOT a classic symptom of MS, and my neuro is very dismissive of it, but I’m intrigued that someone else is reporting the same.
I feel certain the number of joints affected, and to what extent, has increased the longer I’ve had MS (only diagnosed 2 years, but confident I’ve had it much longer than that).
The only explanation I’ve been able to find is that if muscle strength is impaired (due to neurological problems), you may be taking more strain on the tendons instead, and tendons are inclined to snap and pop more.
I’ve got certain big joints (right shoulder is one) that actually “want” to crack. Although not exactly painful, the joint feels sort of uncomfortable and out of kilter. If I move it a certain way, I can engineer a loud “CRACK!”, which brings an immediate sense of release, and even pleasure. The feeling that whatever was not quite right has shifted back into place. But this only lasts a few hours, before it again starts to feel uncomfortable and out-of-place, and I’m seeking that satisfying crack.
I’m sure it all has to do with MS at some level, but nobody medical seems interested, or to be able to explain to me why. 
Tina