air intake noise

i'm new to this board and coming from bimmer.org. I've noticed a gurgling sound coming from the engine compartment. The sound is similar to a cow bell being dragged on the ground, but only appears when I'm accelerating. I took it to the dealer and he said this is normal and that it was the air hitting the manifold. However, when I put the car in park and rev it, this sound does not appear. If it were the air hitting the manifold, wouldn't I hear the sound everytime I pressed on the gas? I'm concerned because I hadn't heard this noise before a couple of weeks ago and the nearest dealer other than mine is 2 hours away. Does anyone have any ideas as to what this noise is. Any help would be appreciated.