it sounds like your dual mass flywheel as the symtoms you describe are common for this type of fault(only occurs when you remove pressure from it when you take foot off acc pedal),if it is it will deteriorate until it vibrates most of the time,it may also be the bolts come loose on the front or side engine mounts as these are also common faults best to get it checked pronto incase bolts are loose,hope this helps.
rich.
January 2012