Can the rubber spring insulator cause a squeak? Not sure. I am just guessing since you say everything was greased 10,000 miles ago. Plus you describe the sound coming from there.
But, after you grease everything and it is still there it is time to remove the spring. Like Woodchuck said. It could also be the rubber on the perch.
As a side note. My bumper bracket arm was a little loose. It also gave an annoying squeak. I know you said it is not a metal sound but easy to check.
'65 Mustang Inline 200. CI Aliminum Head, Holley 350,
Schneider 264/274 cam, Headers, Recurved DSII, MSD 6A, and a whole lotta other stuff!