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: 300E clunking in reverse
Your second statement clounds the issue a bit but try your joints then your motor mounts second.