flex joint most likely. Jack it up and have a look easy diy.
If you can feel the motor at idle and when changing gears could also be you motor mounts. But that is most obvious going into reverse only. Do a search there is loads on this to help you do the diy.
Check out gilly on this thread he tells the way to check your flex joint: http://www.peachparts.com/shopforum/showthread.php?threadid=63395

Your second statement clounds the issue a bit but try your joints then your motor mounts second.
m

__________________
Martin Ingram
Colorado Springs
2005 320 CDI
2006.5 VW Jetta TDI
1991 560SEL (179000 Sold)
1972 280SEL 4.5 ('The Lead Sled' 320000 miles when sold.)
1972 220D (225000 when sold)