Could be a number of things. A loose spark plug, exhaust leak, tensioner pulley etc...
Remove the drive belt and start it up, do whatever you do with the throttle to get it to make the noise and see if it goes away. Could be a pulley bearing, alternator, a/c or what not so eliminate those. Check the plugs to make sure none are loose. They should be torqued to 28ft/lbs. Also, check your pcv valve.
It helps to get someone to do the gas while your under the hood trying to pinpoint the sound with a mechanics stethoscope.