'94 850 GLT here, and I also have a screw or something sliding around in center console. BUT!!! I took mine out (about 15 minutes to figure it out) and the problem is NOT THERE.

I suspect it is a small something 'rother, like a screw, that has gotten into a heater duct. It sounds like metal sliding/rolling on plastic, eh?
I have a neodymium magnet hooked to a wire stuck in the vent under the passenger seat but it hasn't caught the bugger yet. I'll try another enrty spot next.

Now, if you are still intent on removing the console, the tricks are:
Open the armrest, pick out the little cover in the tray - two screws there
Remove the ashtray - two screws with their heads pointing forward by the lighter and under the test point socket cover
Two more screws under in the very front of the console

Volvo did a terrible job designing for manufacture on this - three different screws, three different drivers needed. They really should all be the same.

Chuck