Loud noise coming from underneath car on a 97 E320 (W210)?

Hi,

I have a 1997 E320 with approx. 83k miles on it, For the last 10k miles the car has been making this loud kind of ratling noise, kind of like pipes rattling around, It used to only happen with the a/c on, but now it comes on all the time even without the a/c on, This happens when the car is warm, I have taken it to the dealer on several occasions and they can't seem to identify the problem (Lazy bastards), so this is my last resort, the car was just in for a tune up this past weekend, and the noise still continues, any ideas???

Thanks