I have noticed that when going over bumps at about 30 mph, I hear a little rattle comming from the shocks/struts, the car has about 80k on it. Any idea what could be causing this? My Deville doesn't do this at all.

Check Your Stabilizer Link Bushings For Wear.

I had the same issue with my 96 STS this last winter. From previous experience with struts and strut mounts on several Pontiacs, I figured the rattle maybe coming from the mounts, but not being anything other then an old back yard mechanic I took my STS to a dealership for and Expert opinion. After a several mile test drive the mechanic came back and informed me that indeed the rattle was due to worn struts. 2000 dollars later my STS still rattled and I ended up pouring another 400 dollars into Strut mounts and of course a front-end alignment. Please learn from my experience and have the strut mounts checked out first.

http://www.cadillacforums.com/forums/showthread.php?p=366028#post366028 There's a Technical Service Bulletin for this

Can the strut noise be properly diagnosed when the car is on a rack and the weight is off the wheels?

Can the strut noise be properly diagnosed when the car is on a rack and the weight is off the wheels?

Yes it can. A rubber malet is very handy. I would guess that it was the sway bar (also called stabilizer bar) end links. I just replace mine last week on my eldo with 85k miles on it. If you can't find the source with the car on the rack it is very likely that it is a strut problem.