Noises problem
2002 Infiniti G20 4 cyl Front Wheel Drive Automatic 53000 miles

engine vibrates on startup in park. Noise is coming from driver side of engine. At highway speeds it runs smoothly. Reoccurs intermittently at lower speeds. I jacked up car, got underneath, and pushed up on transmission (on the fluid pan) when noise was happening at this stopped the noise. I can push up the transmission about a 1/4 inch. When I do this, there is a thud sound when I let it down...I also here this thud when I hit a bump when driving. The car has been tuned up and motor mounts have been changed. I also took car to a frame shop and there is no frame damage. Hopefully this is an easy fix, any ideas are appreciated.