Gargling noise when RPM drops...

[ammophobia]

I'm driving a 2007 sedan. I've been having this problem for a while. When releasing the gas pedal while depressing the clutch pedal (so the RPM drops, for example after a short backward/forward motion when parking the car), a gargling noise can be heard (but it's dry so maybe it's not exactly gargling, more like quick, rapid rattling) from inside the cabin, around the center or right-side dash I think, and the noise stops when the engine reaches the idle RPM.

I brought it to the dealership and they couldn't find the source of the noise, and when they tested out another sedan which happens to be on its periodical maintenance, they said it also occurs on that sedan so they assume that it's normal.

I'm extremely dissatisfied by their response. It's pretty annoying because intermittently it will turn from a light gargle to a pretty loud one. Is anyone here experiencing the same thing?

Thanks.