Rep Power: 0

My engine is rattling and shaking the entire car when idle. Any idea?

 
Rep Power: 221

Check engine light?
broken motor mount?

 
Rep Power: 366

hundreds of ideas. Take to a mechanic.

 
Rep Power: 210

Probably engine or transmission mount. On a flat surface put the car in neutral, shut engine off, stand in front of the car and then push the car backwards a few feet. If you hear a metallic tapping noise it's a bad mount. Use a helper if need be to sit in the car and stop the car after it's been pushed a few feet backwards.

Another way to diagnose for mounts is to:
1) have a helper operate the vehicle while you stand to the side of the engine with hood open. Obviously, do this in an open space and not in a garage or near a home (at lease not the first time..lol). With the engine on, e-brake engaged, your left foot pressed on the brake pedal (engaged), put the transmission in forward drive and steadily rev the engine (with your right foot) a bit and hold it there for about five seconds or so. If you hear tapping and/or see excessive lift or shaking from the engine that points to a bad mount. Note that some engine lift is normal so watch some youtube on motor mount testing to help discern. As mentioned the driver/helper will use both feet at the same time to perform this test.
2) repeat step 1, but this time with the transmission in reverse drive.

Any engine misfire or compression issues? As sdaidoji mentioned it could be due to a number of issues.