It's called jute and it's sound deadening and heat insulation.
It's held down with contact cement.
Xylene works well, but I don't know if I'd want a soggy mess of flammable pulp.
Try a wire cup brush or flap wheel in an angle grinder.
Jim,
Lil'Red is a '87 F250 HD, 4.10's, 1356 4x4, Zf-5, 3G, PMGR, Saginaw PS, desmogged with a Holley 80508 and Performer intake.
Too much other stuff to mention.