Does this happen under acceleration? Could it be that your traction control is taking over? Your tires will spin and grab making the car shudder. Is your dash light coming on, indicating power transmission intervention when you feel this? Don't lead foot it, and see if it makes a difference. Also keep it in "D" under these weather conditions since it will then start in 2nd gear. I can't figure out what the noise is, and why it only happens when it rains. It kind of sounds like the car has glass packs on it! It will do it and then stop for a while, and then do it again. Any suggestions would be greatly appreciated Thanks.