If the sound is RMP dependent, meaning if the sound increases in repetition as the RPMs increase then it has to do with engine and engine driven parts. You have to start diagnosing by the process of elimination. Take the bets off and start the car and listen to that same sound that you heard with the belts on. Obviously start the car when its cold so you have a good 10min of safe engine running time before it'll start overheating.
It could also be as simple as an exhaust leak or a loose heat shield.