Going by this image:

It's just about where the line meets the External Oblique muscle. I don't have a clue where it is exactly, but it seems to happen when I do the wall slides only. I think it's my right oblique. It's really hard to pinpoint - I thought it was my latissimus dorsi at first!
It's a sort of a popping/clicking/grinding noise when I lower my right shoulder. When it lowers halfway (my forearm is 90 degrees to my upper arm), I experience it.I don't even know if it's a pain or a discomfort, but I hate it. Doing these wall slides everyday for the last while has helped it somewhat, reducing it from a brief sensation (1 sec ~), it's an instant sensation now that comes and goes. :S
I know it's very hard to imagine, but it's even harder to explain. What do you guys think it could be? I might start doing some soft tissue work.