Clanking sound

Ive got a pretty nasty clanking sound coming from underneath my civic. Its a5 spd and It only happens when I take my foot off the gas but keep it in gear. If I take it out of gear the sound immediately goes away. Being that its related to being in gear I'm assuming this could be something major but I have no idea what would cause this. Anyone have any ideas?

could be clutch or transmission, so yeah id say its probably major

It sounds like something smacking the tranny case? Or is it a growling sort of rattle? Most likley something let go on the clutch or you have a bad bearing.