Does it make this sound only when the swingarm/suspension is flexed while standing still?
Is it a sound like creaking? Does it make this sound when engine cold?
If so, it might be a issue with the rear suspension linkage and/or swingarm bearings lacking grease.
I would put the bike on a stand where you can pull the linkage and check the seals and grease on the rocker arm and link arms.
If the seals got damaged, then water/grit/etc. may have gotten into the bearings.
I know when I checked mine years ago, I had two damaged seals, although the bearings weren't damaged, so I was able to clean and repack after replacing the seals.