CLK 320 coupe vibration

I have a 2003 CLK 320 with 32K on the clock.  There is pronounced vibration when I put pressure on the accelerator between 55 to 65 miles per hour.  I can feel it through the floor pan but not the steering wheel and it goes when I take my foot off the accelerator.   Any ideas folks?  Thanks Weimar

Hi did you ever get to the bottom of the vibration? as i have the same problem.
 
Ta Dave