2000 Ford Focus Vibration when first starting car in the mo

My wife's focus vibrates and the engine is pretty loud when you first start it up when the engine is cool. Once the car has warmed up the noise from the engine calms down signifigantly, but there is still some slight vibration when stopped or idleing. I was told by one person that the fuel injector could be clogged. I bought some fuel injector cleaner and plan on using it. Does that sound right or could it be something else?