It should be pretty smooth. It's probably motor mounts or the air cleaner mount. The air cleaner mount in particular can jackhammer through the whole car. Both being bad would definitely rattle your teeth. I'm sure you'll get three pages of suggestions of what it COULD be, but the only way to know for sure is to do some further investigating. That wouldn't scare me off, certainly. My wagon would barely move under its own power and I wasn't scared off though.

-Rog