Your problem could be anything from an idler pulley going out to your alternator going out or possibley a fan touching the shroud. Try to locate the sound or give us a sound clip. Check your belts for abnormal wear and check for rub spots wear a pulley may be rubbing against the timing cover or engine. A noisy tranny may also be to blame
revs are generally a slight bit higher in park or neutral than when in gear...try this, put in park or neutral then rev it to 1500-2000 then return to idle, if there a change in the sound/pitch of the noise try doing the same with it in gear (foot on the brake & e-brake set) then compare, also have a friend listen under the hood while you do so (a LONG screwdriver can also be used to help pinpoint the location, be sure to be clear of moving parts though, by placing the driver bit end on various places & placing ones ear to the screwdriver handle).