I'd look at two things: The intake gasket as everyone else mentioned and also around the EGR for any leaks. When one of my EGR bolt was loose, i had a dancing idle.

So in other words, some sort of an air leak would cause that. If that is the case for you, then you'd also probably feel some power loss. In my case, I had this sound whenever I accelerated, it was like someone holding a card in the spikes of a bike.