I tend to agree with the previous post. It could be either the throwout bearing or the pilot bearing/bushing.
If it goes away after you let the clutch out or after about 5 seconds with the clutch pedal depressed, I'd tend to look at the pilot bearing/bushing...
because, if it was a throwout bearing problem, it normally would keep doing it as long as the clutch pedal was depressed.
Vibration? Bump steer? Wandering? Read the article (sticky) on Steering, suspension, and driveline.