You should be able to tell whether the noise you are hearing is rattling, or vibration. They could have very different causes.
If you hear rattling somewhere in the dash, then it's probably something in the glovebox or various compartments inside. The first thing I'd do is to take away everything from these and drive around the block for a quick test. Usually it's spare parking change, forgotten CD in the glovebox, or somesuch.
If the dash is vibrating, try pushing it in different places while driving (better yet, ask the passenger next to you to do it while the noise is present to see whether the dash itself is loose). If the noise stops, then you have found the cause. If you have some aftermarket dash-mounted equipment, they might also be responsible (I had a phone mount in my car when I bought it and I needed good 10 minutes to figure out where the vibrations are coming from. With the help of a screwdriver, it's all peace and quiet now).
Same goes if you hear rattling from the back -- make sure everything's secured in the trunk (you could leave the heavier items, they won't rattle), or better yet, take everything away. I'd be more concerned when something rattles on the back, to be honest -- I usually secure everything in the trunk and there shouldn't be anything that rattles at all. Sure, noises from loads adjusting during driving, sharp cornering or bumps are normal, but definitely not constantly.
If the rattle comes from below when you are driving on a bumpy road or cornering a bit hard, then it's most likely the suspension. Nothing to worry about, it could be as simple as anti-roll bar links which you can readily replace yourself. If the rattling is not constant, I'd wait with the repair until it is -- it will be easier to identify what's wrong anyway. But if you have problems with steering or the suspension is wobbly -- I'd definitely check that out immediately.