A challenge regarding back pain for any would be specialists out there

Here is a challenge for anyone brave enough to take a stab at it based on a verbal description!!

A reoccurring muscle knot / soft tissue mass located in the lower left erector spinae / quadratus lumborum area.
I say area as sometimes it will be located IN the erector spinae, and sometimes just behind it.
The mass / knot is palpable, however, can be very hard to "break up" if at all.
No action / exercise seems to be able to bring on the pain on cue, that is, I can run and run for a week with no issues and then on a rest day, there it is!
The next month may be the opposite.
X-rays, MRIs and CTs have shown nothing sinister.
Countless professionals have not been able to correctly diagnose the issue - with some suggestions relating to a hypomobile SIJ, tight hamstrings, tight hips, poor core strength and even once I was told it was down to an emotional gall bladder!
The pain does not travel away from the area, but does make the whole of my lower left back tight.
Deep long stretching has limited success at time.
Deep heat does tend to take the pain sensation away very temporarily, but that is probably due to the methyl salicylate no doubt.
I have noticed that my right glute tends to feel less bulky than my left, and also feels tighter when stretching
I am a fit, athletically built individual with nothing but the odd niggle / injury from running in terms of aliments.

Any thoughts on this would be very much appreciated as I have spent a tremendous amount of time, effort, energy and money in trying to find the answer to my issue over 8 or 9 years - with absolutely no success what so ever.
So if anyone out there fancies a challenge - fire away - if not - I think I'm going to freeze myself until they have developed robotic replacements for every muscle and bone in the human body.

Thanks,

Alex