There are several possibilities for the noise you are hearing...
(1) If it sounds like a metallic rattling noise originating underneath the vehicle, it could be from a loose exhaust heat shield. Typically this will sound like a tinny, sheet metal kind of rattle.
(2) If the noise is louder than that, you may be hearing an exhaust leak from a rusted or broken exhaust component, or a cracked exhaust manifold. Sometimes this type of noise may sound louder under acceleration, due to an increase in exhaust flow and/ or engine movement causing a loose joint to open slightly.
(3) If the noise is engine related and sounds like a handfull of marbles rattling around under the hood, and only occurs on acceleration or under load such as when climbing hills, you may be hearing detonation. This occurs when the fuel charge ignites before the spark plug fires due to too high a cylinder temperature, incorrect ignition timing, or excessive carbon deposits in a cylinder.
Having a noise such as you describe diagnosed at a repair shop usually does involve much time, and therefore does not usually cost very much, if you are unable to determine the source of the complaint.
I hope this is helpful; if so, an accept would be most appreciated! Thanks!