Coil spring.

And everything I know about cars I know from Haynes manuals and Tom And Ray. But this noise fits into the "loud and potentially violent sounding" territory. Doesn't look like it would be that hard to fix if I knew how to detatch a few things, and had a garage, I'd just pull the whole shock... "column" is the best word I can come up with, don't know the right one. Unbolt it from up top, and swap in used parts from a bone yard. But I figure if one is getting bad, get the other one checked.