I had an annoying rattle when travelling over bumpy roads, and suspected the plastic pan under the engine compartment. Anyway the rattle seems to have gone now that I've removed the pan. Is any important function served by that pan? If so, is there any way to eliminate the rattle (the bolts were all in tightly)?

I think the S60 with Standard transmissions do not have the belly pan cover while the auto trans do. That tells me something. It must be there to divert airflow over critical components that may need additional cooling. There is a 'scoop' in the middle of the pan. I would not leave the pan off.
Regards,
Eddie W. in FL

Either the pan was loose, which you said it wasnt, or something was contacting it. Did you notice any wear spots on it where something might have been making contact? If so, maybe that will lead you to the source of the rattle. Then again, maybe it wasnt even that, and the rattle has mysteriously gone away. Try putting it back on and see if it reappears...etc.

Before I removed the pan, I checked two things:
1) All the bolts were tight.
2) Tapping on the pan caused it to rattle against the metal of the chassis.

Maybe the pan is warped a bit? But do you know how important that pan is? Exactly what is it supposed to protect? No previous car I've owned has ever had such a pan.

I personally dont think it will "harm" anything by removing it. On the other hand, I do know that manufacturers like to save money any way they can, and putting a "needless" pan under the car would seem silly. So my guess is that they have a "reason", whatever it may be. Some guesses would be to protect some fragile component that is now located somewhere near there, or to increase/decrease airflow in the engine compartment. Who knows.