It's wheel hop and engine movement. One of the biggest issues from the factory with our cars is the squishy engine mounts that allow a massive amount of engine movement to provide a smoother ride with less vibrations. Upgrading the rear engine mount is one of the easiest and cheapest mods a lot of us has done. It helps with the wheel hop and engine movement, but not drastically. I'm still waiting (nay, hoping) that someone comes out with a full set of engine mount inserts for our cars, like what I have for my '03 SpecV. That helped tremendously with these issues, but does transfer a bit more vibration to the cabin, especially via the shifter (if you have a manual), and the pedals.
Just be forewarned, while it's fairly uncommon, a lot of hard, repetitive beating on the engine mounts can make them fail, and fail hard.