Guys, help.

Yung engine kakaiba yung vibration nya when running at high speed or around 4th and 5th gear.
its kinda violent na I can feel it shaking my ride compared to normal engine vibration.\
then ang pag nag shake na sya, bibitawan ko lang yung gas and go to neutral, magiging normal na ulit
wala naman weird noise. ok din ang engine rhythm pag low speeds; 1st 2nd 3rd gear.

my engine is 4d56, mitsu adventure 1st generation

any idea anong problem? THANKS!