I think everyone is thinking of the worst possible situation. Keep it simple stupid. Start with the basics first. If it was me, I would either try to get the vehicle on a hoist or jack up the car far enough so that someone can get underneath it, then get someone else to hold the car at the rpm where you hear the rattle, then get the person who's underneath the car to try and locate the rattle. Could be something simple as a loose exhaust heat shield or something like that viabrating against something.
If that brings up nothing I'd then take it to a decent mechanic, they should be able to find what's wrong and if it is something serious like bearings in the motor or whatnot they will be able to give you a quote of how much to fix.
Cheers,
Mike.