Is it a tinny sound, like marbles in a coffee can? Are you old enough to remember when coffee came in a metal can?
Your noise could possibly be a loose catalytic converter heat shield. It's not uncommon for a weld or two on the heat shield to crack causing a rattle at certain engine speeds. The easy fix is to get one of the huge hose clamps and place it around the heat shield. Most parts stores will probably know what you need if you ask for a catalytic converter heat shield hose clamp.
Don't remove the heat shield as they are there for a very good reason: to keep your car from igniting the insulation on the floorboard as well as that pile of leaves you drove through on your way to work.
-Rod