Linda,Does the steering wheel vibrate, does the front end of the vehicle vibrate or does the drivers seat vibrate when this happens? There could be a ton of things that can cause a vibration. If it is happening at idle in park, then I would suspect something engine related. If this happens at 60 mph or on the expressway, then I would suspect a bad tire or rim. It is possible that you have a bend rim or a belt shift in one of your tires.But if the problem is only at idle, I would suspect something with the timing belt, an air or vacuum leak or something with the crankshaft pulley (harmonic balancer) but I think you said that was replaced. If so, I would have them double check the timing belt and check for air and vacuum leaks. Another possible cause of a vibration issue would be an axle shaft. This is also pretty common but again, this would not cause a vibration in park because the drive shaft would need to be turning in order to cause a vibration. But this could in fact cause a vibration at 20 or 30 or even at higher speeds 50 ot 60 mph. I hope this helps! Please let me know if you have any further questions!