bad engine mount maybe?
 

on my 300sd the air cleaner raddles really bad at idle when in gear. if i put it in nuteral it stops raddling. i know its not the mounts for the air cleaner so what else could it be? engine mounts,engine shocks? any info would be appreciated.

Maybe all of the above and some more. Check under the air cannister itself. There are three small rubber mounts. Could be one or more are broke. Also possible the bracket holding the air cannister itself is broke.
If they are all ok, and the air can just rattles because your engine is not in perfect running order, or because the rest of the air cannister assembly is just worn a bit and is loose, there is a simple fix.
You can get some weather seal at a local hardware and tape a small section to the air cannister where is bangs into the valve cover. Or some have used on of the busted rubber mounts under the assembly and drilled a hole in the side of the air cannister assembly and mounted that part there to prevent the ever annoying rattle.

i figure out the problem. one of the bolts on one of the little mounts pulled through the threds on the mounting bracket. simple fix. i put a nut on it