The sound of engine is not smooth (up and down) when it stops.

I have 2007 Ford Falcon, 4.0 L, V6, sedan. The sound of the engine is not smooth, when it drives, it is fine, but when it stops, the sound is regularly intermittent. The sound is really regular, kind of regular sound breaking, not smooth. The sound is not smooth and constant, rather up and down a little bit, mainly when it stops. Recently, done the servicing for the 75K from the ford dealer. They dint get any problem... any idea, from where it may be coming..
Thanks for your help.