Rear suspension groan

I’m new to this forum even though I have had my 2004 NB 2.5 Miata for almost 3 years already. The intent is to restore it as a “retirement project” while having as much fun as possible given the somewhat more time available to do things that make sense and not chase the corporate dream.

Of late I get a groaning noise form the rear suspension when I brake. I have checked the brake pads and rotors and these are all fine - and anyway it is not a grinding noise.

Any ideas where I should start looking?

Pierre