Have you tried a layer of "gorilla tape" on the inner joint surface?

I was getting creaks on my jetstream and put some "engineering marker" on both hinge surfaces to highlight where the rubbing/creaking was coming from. As the frame hinge features were not machined to a high level on precision with some frame paint on top and mechanism stacked up tolerances rather large... there are at least micro-motions resulting in creaking.
After a few rides, it showed where the frame was in contact/compression and highlighted the high and low spots...
I trimmed a piece of gorilla tape, stuck in on the inner surface on the frame and as the hinge closed and went compression, the tape layer added an extra "soft-clamping" load ; it filled the gaps and dampened the system. Not more creak and micromotion. Note that it is classic "trick" to stop buzz in race cars.

I did the same on the Helios.