1996 Saturn SL2 Loud vibration @ start-up and deceleration.

Noises problem
1996 Saturn SL2 4 cyl Manual 90000 miles

My car has a loud vibration that sounds like it is coming from the engine towards the passenger side floor board. The vibration is worse at times and sometimes non-existent. It tends to always vibrate whenever I start my car and the RPM's are higher while the car warms up. It also vibrates more during left turns than right turns. If I am driving on the highway and suddenly decelerate the vibration is very bad unless I shift into nuetral in which case the vibration is much more subdued.
I've had people tell me everything from vacuum leaks to engine mounts, or my transmission. Please help me.