When you are trying to fast shift, especially in low gears normal driving, you are rocking your motor mounts and transmission mounts. What you are feeling is, your whole engine and transmission rocking, ever heard the engines exhaust as you do it? It makes the baaa baa baaaaaa, like somewhat of a rocking sound.
That is a good way to replace new motor mounts with new motor mounts
. 1st through 3rd gear are very shorts gears, torquey. Whatever you want to call it. Smooth shifts, wait for rpms to go down before shifting or buy racing mounts