I have a '87 420sel which was running fine until last week. It now runs rough enough at idle to shake the car. The car runs fine at speed. There is a noticable (loud) hissing sound coming from (seemingly) under the air cleaner. I looked around the engine for any loose or cracked hoses, etc. but found none. Upon removal of the air cleaner, I believe the hiss is coming from, bear with me here, the "circular thing" that opens when the engine is revved and closes at idle.

Sorry to be so technical, but hopefully you understand what I'm getting at.