97 Maxima 3.0 with 80K QUESTION

Hello all, I just wanted to know if its normal to hear like air being pumped into the manifold from the emission control system--when i rev up the engine while the hood is open.. I dont hear this noise when driving the car only when i rev it a bit with the hood open. The sound is near the fire wall but down on the engine... Everything is dry and clean-no exh leaks and teh car runs great..I'm just a fanatic with things and today i heard that sound.. Thanks for any help........ MARK