Just bought a 2000 s70 with 70,000 and "discovered" a rattle in the front end when going over even smaller bumps. Don't thing its the spring seats but could be. The plastic tray under the front engine seems to be loose and knock against something when I push it up. Are these things normally so loose? Any other thoughts. Also there is a bubbling noise behind the front dash when I first turn the heat on but seems to go away after the car heats up. Could this be air in the coolant lines. Just had the coolant replaced.

I had a 850 before this car and it seemed a lot quieter going over bumps and a little tighter feel to the drive??? Maybe I'm paranoid.

Stabilizer links, aka 'sway bar links' would be very suspect from your description. They connect the stabilzer bar to the lower control arm and their bushings wear out. They are a wear part, usually lasting about 30k miles depending on road quality in your area, and will make small bump-bump or thunk-thunk as you go over undulations that move the front suspension control arms. Cheap and quick to replace. I think the part is about $25 each.

I had a similar rattle on my S80 which drove me nuts. Remove the pan and see if it cures it. If so, here's what I did. There are several points near the rear where the panel can hit on the frame. A few raps on the bottom will disclose these. I took some scraps of old indoor/outdoor carpet and pop-rivited them to the pan over the contact points. Cured!!!
Air in the coolant lines will work its way out in time.